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      WIDTH="72" HEIGHT=48></A> REC-smil-19980615
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<H1 class=maintitle align=center STYLE="text-align: center">
  Synchronized Multimedia Integration Language (SMIL) 1.0 Specification
</H1>
<P ALIGN=Center>
<STRONG>W3C Recommendation 15-June-1998</STRONG>
<P ALIGN=Center>
<DL>
  <DT>
    This version:
  <DD>
    <A HREF="http://www.w3.org/TR/1998/REC-smil-19980615">http://www.w3.org/TR/1998/REC-smil-19980615</A>
  <DT>
    Latest version:
  <DD>
    <A HREF="http://www.w3.org/TR/REC-smil">http://www.w3.org/TR/REC-smil</A>
  <DT>
    Previous version:
  <DD>
    <A HREF="http://www.w3.org/TR/1998/PR-smil-19980409">http://www.w3.org/TR/1998/PR-smil-19980409</A>
  <DT>
</DL>
<P>
<SMALL><A href="/Consortium/Legal/ipr-notice#Copyright">Copyright</A>
&copy; 1998 <A href="http://www.w3.org/">W3C</A>
(<A href="http://www.lcs.mit.edu/">MIT</A>,
<A href="http://www.inria.fr/">INRIA</A>,
<A href="http://www.keio.ac.jp/">Keio</A>), All Rights Reserved. W3C
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<H2>
  About this Document
</H2>
<P>
This document has been prepared by the Synchronized Multimedia Working Group
(WG) of the World Wide Web Consortium. The WG included the following individuals:
<UL>
  <LI>
    Stephan Bugaj, Lucent/Bell Labs
  <LI>
    Dick Bulterman, CWI
  <LI>
    Bruce Butterfield, RealNetworks
  <LI>
    Wo Chang, NIST
  <LI>
    Guy Fouquet, Alcatel
  <LI>
    Christian Gran, GMD
  <LI>
    Mark Hakkinen, The Productivity Works
  <LI>
    Lynda Hardman, CWI
  <LI>
    Peter Hoddie, Apple
  <LI>
    Klaus Hofrichter, GMD
  <LI>
    Philipp Hoschka, W3C
  <LI>
    Jack Jansen, CWI
  <LI>
    George Kerscher, DAISY Consortium
  <LI>
    Rob Lanphier, RealNetworks
  <LI>
    Nabil Laya&iuml;da, INRIA
  <LI>
    Stephanie Leif, RealNetworks
  <LI>
    Sjoerd Mullender, CWI
  <LI>
    Didier Pillet, CNET/DSM
  <LI>
    Anup Rao, Netscape
  <LI>
    Lloyd Rutledge, CWI
  <LI>
    Patrick Soquet, Havas
  <LI>
    Warner ten Kate, Philips
  <LI>
    Jacco van Ossenbruggen, CWI
  <LI>
    Michael Vernick, Lucent/Bell Labs
  <LI>
    Jin Yu, DEC
</UL>
<P>
<EM><B>Acknowledgments:</B></EM> In addition to the working group members,
the following people contributed to the SMIL effort: Bert Bos (W3C), Dan
Connolly (W3C), Patrick Deunhouwer (Philips), Martin D&uuml;rst (W3C), Al
&nbsp;Gilman, H&aring;kon Lie (W3C), Chris Lilley (W3C), Curtis Reynolds
(RealNetworks), Michael Riesman, Curtis Reynolds (RealNetworks), Henning
Schulzrinne (Columbia University) and Koga Youichirou (W3C).
<P>
<B><I>Editor: </I></B>Philipp Hoschka, W3C
(<A HREF="mailto:hoschka@w3.org">hoschka@w3.org</A>)
<H2>
  Abstract
</H2>
<P>
This document specifies version 1 of the Synchronized Multimedia Integration
Language (SMIL 1.0, pronounced "smile"). SMIL allows integrating a set of
independent multimedia objects into a synchronized multimedia presentation.
Using SMIL, an author can
<OL>
  <LI>
    describe the temporal behavior of the presentation
  <LI>
    describe the layout of the presentation on a screen
  <LI>
    associate hyperlinks with media objects
</OL>
<P>
This specification is structured as follows: Section 1 presents the specification
approach. Section 2 defines the "smil" element. Section 3 defines the elements
&nbsp;that can be contained in the head part of a SMIL document. Section
4 defines the elements that can be contained in the body part of a SMIL document.
In particular, this Section defines the time model used in SMIL. Section
5 describes the SMIL DTD.
<H2>
  Status of this Document
</H2>
<P>
This document has been reviewed by W3C Members and other interested parties
and has been endorsed by the Director as a W3C Recommendation. It is a stable
document and may be used as reference material or cited as a normative reference
from another document. W3C's role in making the Recommendation is to draw
attention to the specification and to promote its widespread deployment.
This enhances the functionality and interoperability of the Web.
<P>
Comments on this Recommendation may be sent to the <A HREF="http://lists.w3.org/Archives/Public/www-smil/">public mailing list</A>
<A HREF="mailto:www-smil@w3.org">www-smil@w3.org</A>.
<H2>
  Available languages
</H2>
<P>
The English version of this specification is the only normative version.
However, for translations in other languages see
<A HREF="/AudioVideo/SMIL/translations">http://www.w3.org/AudioVideo/SMIL/translations</A>.
<H2>
  Errata
</H2>
<P>
The list of known errors in this specification is available at
<A HREF="/AudioVideo/SMIL/errata">http://www.w3.org/AudioVideo/SMIL/errata</A>.
<H2>
  Table of Contents
</H2>
<UL>
  <LI>
    <A HREF="#specification">1 Specification Approach</A>
  <LI>
    <A HREF="#smil">2 The smil Element</A>
  <LI>
    <A HREF="#document-head">3 The Document Head</A>
    <UL>
      <LI>
	<A href="#head">3.1 The head Element</A>
      <LI>
	<A HREF="#layout">3.2 The layout Element</A>
      <LI>
	<A HREF="#layout-elements">3.3 SMIL Basic Layout Language</A>
	<UL>
	  <LI>
	    <A href="#region">3.3.1 The region Element</A>
	  <LI>
	    <A href="#root-layout">3.3.2 The root-layout &nbsp;Element</A>
	</UL>
      <LI>
	<A href="#meta">3.4 The meta Element</A>
    </UL>
  <LI>
    <A HREF="#document-body">4 The Document Body</A>
    <UL>
      <LI>
	<A HREF="#body">4.1 The body Element</A>
      <LI>
	<A href="#synchronization-elements">4.2 Synchronization Elements</A>
	<UL>
	  <LI>
	    <A HREF="#par">4.2.1 The par Element</A>
	  <LI>
	    <A HREF="#seq">4.2.2 The seq Element</A>
	  <LI>
	    <A HREF="#media-object">4.2.3 Media Object Element</A>: The ref, animation,
	    audio, img, video, text and textstream elements
	  <LI>
	    <A HREF="#SMIL_Time_Model">4.2.4 SMIL Time Model</A>
	    <UL>
	      <LI>
		<A HREF="#Time_Model_Variables">4.2.4.1 Time Model Values</A>
	      <LI>
		<A HREF="#Determining_Values">4.2.4.2 Determining Values of Model Values
		for SMIL 1.0 Elements</A>
	    </UL>
	</UL>
      <LI>
	<A HREF="#switch">4.3 The switch Element</A>
      <LI>
	<A HREF="#test">4.4 Test Attributes</A>
      <LI>
	<A HREF="#hyperlinking">4.5 Hyperlinking</A> Elements
	<UL>
	  <LI>
	    <A HREF="#a">4.5.1 The a Element</A>
	  <LI>
	    <A HREF="#anchor">4.5.2 The anchor Element</A>
	</UL>
    </UL>
  <LI>
    <A HREF="#smil-dtd">5 SMIL DTD</A>
    <UL>
      <LI>
	<A HREF="#relation">5.1 Relation to XML</A>
      <LI>
	<A HREF="#dtd">5.2 DTD</A>
    </UL>
  <LI>
    <A HREF="#Appendix">Appendix</A>
    <UL>
      <LI>
	<A HREF="#handling-extensions">Extending SMIL 1.0</A>
      <LI>
	<A HREF="#Using">Using SMIL 1.0 as an Extension</A>
    </UL>
</UL>
<H2>
  <A name="specification">1 Specification Approach</A>
</H2>
<P>
SMIL documents are XML 1.0 documents <A HREF="#ref-XML10">[XML10]</A>. The
reader is expected to be familiar with the concepts and terms defined in
XML 1.0.
<P>
This specification does not rely on particular features defined in URLs that
cannot potentially be expressed using URNs. Therefore, the more generic term
URI <A HREF="#ref-URI">[URI] </A>is used throughout the specification.
<P>
The syntax of SMIL documents is defined by the DTD in <A HREF="#dtd">Section
5.2</A>. The syntax of an attribute value that cannot be defined using the
DTD notation is defined together with the first element using an attribute
that can contain the attribute value. The syntax of such attribute values
is defined using the Extended Backus-Naur Form (EBNF) defined in the XML
1.0 specification.
<P>
An element definition is structured as follows: First, all attributes of
the element are defined in alphabetical order. An attribute is defined in
the following way: If the attribute is used by an element for the first time
in the specification, the semantics of the attribute are defined. If the
attribute has already been used by another element, the specification refers
to the definition of the attribute in the first element that used it. The
definition of element attributes is followed by the definition of any attribute
values whose syntax cannot be defined using the DTD notation. &nbsp;The final
section in an element definition specifies the element content.
<H2>
  <A name="smil">2 The <TT>smil</TT> Element</A>
</H2>
<P>
<STRONG>Element Attributes</STRONG>
<P>
The "smil" element can have the following attribute:
<DL>
  <DT>
    <A NAME="id">id</A>
  <DD>
    This attribute uniquely identifies an element within a document. Its value
    is an XML identifier.
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
The "smil" element can contain the following children:
<DL>
  <DT>
    body
  <DD>
    Defined in <A HREF="#body">Section 4.1</A>
  <DT>
    head
  <DD>
    Defined in <A HREF="#head">Section 3.1</A>
</DL>
<H2>
  <A NAME="document-head">3 The Document Head</A>
</H2>
<H3>
  <A NAME="head">3.1 </A>The <CODE>head</CODE> Element
</H3>
<P>
The "head" element contains information that is not related to the temporal
behavior of the presentation.
<P>
<STRONG>Element Attributes</STRONG>
<P>
The "head" element can have the following attribute:
<DL>
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A>
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
The "head" element can contain the following children:
<DL>
  <DT>
    layout
  <DD>
    Defined in <A HREF="#layout">Section 3.2</A>
  <DT>
    meta
  <DD>
    Defined in <A HREF="#meta">Section 3.4</A>
  <DT>
    switch
  <DD>
    Defined in <A HREF="#switch">Section 4.3</A>
</DL>
<P>
The "head" element may contain any number of "meta" elements and either a
"layout" element or a "switch" element.
<H3>
  <A name="layout">3.2</A> The <TT>layout</TT> Element
</H3>
<P>
The "layout" element determines how the elements in the document's body are
positioned on an abstract rendering surface (either visual or acoustic).
<P>
If a document contains no layout element, the positioning of the body elements
is implementation-dependent.
<P>
A SMIL document can contain multiple alternative layouts by enclosing several
layout elements within a "switch" element (defined in <A HREF="#switch">Section
4.3</A>). This can be used for example to describe the document's layout
using different layout languages.
<P>
The following example shows how CSS2 can be used as alternative to the SMIL
basic layout language (defined in <A HREF="#layout-elements">Section 3.3</A>):
<PRE>&lt;smil&gt;
&nbsp; &lt;head&gt;
&nbsp; &nbsp; &lt;switch&gt;
&nbsp; &nbsp; &nbsp;&lt;layout type="text/css"&gt;
        [region="r"] { top: 20px; left: 20px }
     &lt;/layout&gt;
&nbsp; &nbsp; &nbsp;&lt;layout&gt;
&nbsp; &nbsp; &nbsp;  &lt;region id="r" top="20" left="20" /&gt;
&nbsp; &nbsp; &nbsp;&lt;/layout&gt;
   &lt;/switch&gt;
   &lt;/head&gt;
   &lt;body&gt;
     &lt;seq&gt;
       &lt;img region="r" src="http://www.w3.org/test" dur="10s" /&gt;
     &lt;/seq&gt;
   &lt;/body&gt;
&lt;/smil&gt;
</PRE>
<P>
(note that in this example, both layout alternatives result in the same layout)
<P>
<STRONG>Element Attributes </STRONG>
<DL>
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A>
  <DT>
    <A NAME="layout-type">type</A>
  <DD>
    This attribute specifies which layout language is used in the layout element.
    If the player does not understand this language, it must skip all content
    up until the next "&lt;/layout&gt;" tag. The default value of the type attribute
    is "text/smil-basic-layout".
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
If the type attribute of the layout element has the value
"text/smil-basic-layout", it can contain the following elements:
<DL>
  <DT>
    region
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A>
  <DT>
    root-layout
  <DD>
    Defined in <A HREF="#root-layout">Section 3.3.2</A>
</DL>
<P>
If the type attribute of the "layout" element has another value, the element
contains character data.
<H3>
  <A name="layout-elements">3.3 SMIL Basic Layout Language</A>
</H3>
<P>
This section defines a basic layout language for SMIL. SMIL basic layout
is consistent with the visual rendering model defined in CSS2, it reuses
the formatting properties defined by the CSS2 specification, and newly introduces
the "fit" attribute <A HREF="#ref-CSS2">[CSS2]</A>. The reader is expected
to be familiar with the concepts and terms defined in CSS2.
<P>
SMIL basic layout only controls the layout of media object elements (defined
in <A HREF="#media-object">Section 4.2.3</A>). It is illegal to use SMIL
basic layout for other SMIL elements.
<P>
The type identifier for SMIL basic layout is "text/smil-basic-layout".
<P>
<STRONG>Fixed Property Values</STRONG>
<P>
The following stylesheet defines the values of the CSS2 properties "display"
and "position" that are valid in SMIL basic layout. These property values
are fixed:
<PRE>a           {display:block}
anchor      {display:block}
animation   {display: block;
             position: absolute}
body        {display: block}
head        {display: none}
img         {display: block;
             position: absolute}
layout      {display: none}
meta        {display: none}
par         {display: block}
region      {display: none}
ref	    {display: block;
             position: absolute}
root-layout {display: none}
seq         {display: block}
smil        {display: block}
switch      {display:block}
text        {display: block;
             position: absolute}
textstream  {display: block;
             position: absolute}
video       {display: block;
             position: absolute}
</PRE>
<P>
Note that as a result of these definitions, all absolutely positioned elements
(animation, img, ref, text, textstream and video) are contained within a
single containing block defined by the content content edge of the root element
(smil).
<P>
<STRONG>Default Values</STRONG>
<P>
SMIL basic layout defines default values for all layout-related attributes.
These are consistent with the initial values of the corresponding properties
in CSS2.
<P>
If the author wants to select the default layout values for <EM>all</EM>
media object elements in a document, the document must contain an empty layout
element of type "text/smil-basic-layout" such as:
<PRE>&lt;layout type="text/smil-basic-layout"&gt;&lt;/layout&gt;
</PRE>
<H4>
  <A name="region">3.3.1 <STRONG>The <TT>region </TT>Element</STRONG></A>
</H4>
<P>
The region element controls the position, size and scaling of media object
elements.
<P>
In the following example fragment, the position of a text element is set
to a 5 pixel distance from the top border of the rendering window:
<PRE>&lt;smil&gt;
&nbsp; &lt;head&gt;
    &lt;layout&gt;
&nbsp; &nbsp;   &lt;region id="a" top="5" /&gt;
    &lt;/layout&gt;
&nbsp; &lt;/head&gt;
&nbsp; &lt;body&gt;
&nbsp; &nbsp; &lt;text region="a" src="text.html" dur="10s" /&gt;
&nbsp; &lt;/body&gt;
&lt;/smil&gt;
</PRE>
<P>
<STRONG>Element &nbsp;Attributes </STRONG>
<P>
The "region" element can have the following attributes:
<DL>
  <DT>
    background-color
  <DD>
    The use and definition of this attribute are identical to the "background-color"
    property in the CSS2 specification, except that SMIL basic layout does not
    require support for "system colors". <BR>
    If the background-color attribute is absent, the background is transparent.
  <DT>
    <A NAME="scale">fit</A>
  <DD>
    This attribute specifies the behavior if the intrinsic height and width of
    a visual media object differ from the values specified by the height and
    width attributes in the "region" element. This attribute does not have a
    1-1 mapping onto a CSS2 property, but can be simulated in CSS2.<BR>
    This attribute can have the following values:
    <DL>
      <DT>
	fill
      <DD>
	Scale the object's height and width independently so that the content just
	touches all edges of the box.
      <DT>
	hidden
      <DD>
	<UL>
	  <LI>
	    If the intrinsic height (width) of the media object element is smaller than
	    the height (width) defined in the "region" element, render the object starting
	    from the top (left) edge and fill up the remaining height (width) with the
	    background color.
	  <LI>
	    If the intrinsic height (width) of the media object element is greater than
	    the height (width) defined in the "region" element, render the object starting
	    from the top (left) edge until the height (width) defined in the "region"
	    element is reached, and clip the parts of the object below (right of) the
	    height (width).
	</UL>
      <DT>
	meet
      <DD>
	Scale the visual media object while preserving its aspect ratio until its
	height or width is equal to the value specified by the height or width
	attributes, while none of the content is clipped. The object's left top corner
	is positioned at the top-left coordinates of the box, and empty space at
	the left or bottom is filled up with the background color.
      <DT>
	scroll
      <DD>
	A scrolling mechanism should be invoked when the element's rendered contents
	exceed its bounds.
      <DT>
	slice
      <DD>
	Scale the visual media object while preserving its aspect ratio so that its
	height or width are equal to the value specified by the height and width
	attributes while some of the content may get clipped. Depending on the exact
	situation, either a horizontal or a vertical slice of the visual media object
	is displayed. Overflow width is clipped from the right of the media object.
	Overflow height is clipped from the bottom of the media object.
    </DL>
    <P>
    The default value of "fill" is "hidden".
  <DT>
    <A NAME="height">height</A>
  <DD>
    The use and definition of this attribute are identical to&nbsp;the "height"
    property in the CSS2 specification. Attribute values can be "percentage"
    values, and a variation of the "length" values defined in CSS2. For "length"
    values, SMIL basic layout only supports pixel units as defined in CSS2. It
    allows to leave out the "px" unit qualifier in pixel values (the "px" qualifier
    is required in CSS2).
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A><BR>
    A region element is applied to a positionable element by setting the
    <A HREF="#loc">region</A> attribute of the positionable element to the id
    value of the region.<BR>
    The "id" attribute is required for "region" elements.
  <DT>
    <A NAME="left">left</A>
  <DD>
    The use and definition of this attribute are identical to&nbsp;the "left"
    property in the CSS2 specification. Attribute values have the same restrictions
    as the attribute values of the "height" attribute.<BR>
    The default value is zero.
  <DT>
    skip-content
  <DD>
    This attribute is introduced for future extensibility of SMIL (see
    <A HREF="#Appendix">Appendix</A>). It is interpreted in the following two
    cases:
    <UL>
      <LI>
	If a new element is introduced in a&nbsp;future version of SMIL, and this
	element allows SMIL 1.0 elements as element content, the "skip-content" attribute
	controls whether this content is processed by a SMIL 1.0 player.
      <LI>
	If an empty element in SMIL version 1.0 becomes non-empty in a future SMIL
	version, the "skip-content" attribute controls whether this content is ignored
	by a SMIL 1.0 player, or results in a syntax error.
    </UL>
    <P>
    If the value of the "skip-content" attribute is "true", and one of the cases
    above apply, the content of the element is ignored. If the value is "false",
    the content of the element is processed. <BR>
    The default value for &nbsp;"skip-content" is "true".
  <DT>
    title
  <DD>
    This attribute offers advisory information about the element for which it
    is set. Values of the title attribute may be rendered by user agents in a
    variety of ways. For instance, visual browsers frequently display the title
    as a "tool tip" (a short message that appears when the pointing device pauses
    over an object). <FONT COLOR="Red"><B><BR>
    </B></FONT>It &nbsp;is strongly recommended that all "region" elements have
    a "title" attribute with a meaningful description. Authoring tools should
    ensure that no element can be introduced into a SMIL document without this
    attribute.
  <DT>
    <A NAME="top">top</A>
  <DD>
    The use and definition of this attribute are identical to&nbsp;the "top"
    property in the CSS2 specification. Attribute values have the same restrictions
    as the attribute values of the "height" attribute.<BR>
    The default value is zero.
  <DT>
    <A NAME="width">width</A>
  <DD>
    The use and definition of this attribute are identical to&nbsp;the "width"
    property in the CSS2 specification. Attribute values have the same restrictions
    as the attribute values of the "height" attribute.
  <DT>
    z-index
  <DD>
    The use and definition of this attribute are identical to&nbsp;the "z-index"
    property in the CSS2 specification, with the following exception:
</DL>
<DL>
  <DD>
    <UL>
      <LI>
	If two boxes<FONT COLOR="Red"> </FONT>generated by elements A and B have
	the same stack level, then
	<OL>
	  <LI>
	    If the display of an element A starts later than the display of an element
	    B, the box of A is stacked on top of the box of B (temporal order).
	  <LI>
	    If the display of the elements starts at the same time, and an element A
	    occurs later in the SMIL document text than an element B, the box of A is
	    stacked on top of the box of B (document tree order as defined in CSS2).
	</OL>
    </UL>
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
"region" is an empty element.
<H4>
  <A name="root-layout">3.3.2 The <CODE>root-layout</CODE> element</A>
</H4>
<P>
The "root-layout" element determines the value of the layout properties of
the root element, which in turn determines the size of the viewport, e.g.
the window in which the SMIL presentation is rendered.
<P>
If a document contains more than one "root-layout" element, this is an error,
and the document should not be displayed.
<P>
<STRONG>Element Attributes </STRONG>
<P>
The "root-layout" element can have the following attributes:
<DL>
  <DT>
    background-color
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A>
  <DT>
    height
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A><BR>
    Sets the height of the root element. Only length values are allowed.
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A>
  <DT>
    skip-content
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A>
  <DT>
    title
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A>
  <DT>
    width
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A><BR>
    Sets the width of the root element. Only length values are allowed.
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
"root-layout" is an empty element.
<H3>
  <A NAME="meta">3.4 The <TT>meta</TT> Element</A>
</H3>
<P>
The "meta" element can be used to define properties of a document (e.g.,
author, expiration date, a list of key words, etc.) and assign values to
those properties. Each "meta" element specifies a single property/value pair.
<P>
<STRONG>Element Attributes</STRONG>
<P>
The "meta" element can have the following attributes:
<DL>
  <DT>
    content
  <DD>
    This attribute specifies the value of the property defined in the meta
    element.<FONT COLOR="Red"><BR>
    </FONT>The "content" attribute is required for "meta" elements.
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A>
  <DT>
    name
  <DD>
    This attribute identifies the property defined in the meta
    element.<FONT COLOR="Red"><BR>
    </FONT>The "name" attribute is required for "meta" elements.
  <DT>
    skip-content
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A>
</DL>
<P>
The list of properties is open-ended. This specification defines the following
properties:
<DL>
  <DT>
    base
  <DD>
    The value of this property determines the base URI for all relative URIs
    used in the document.
  <DT>
    pics-label or PICS-Label
  <DD>
    The value of this property specifies a valid rating label for the document
    as defined by PICS <A HREF="#ref-PICS">[PICS]</A>.
  <DT>
    title
  <DD>
    The value of this property contains the title of the presentation.
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
"meta" is an empty element.
<H2>
  <A name="document-body">4 The Document Body</A>
</H2>
<H3>
  <A NAME="body">4.1</A> The <CODE>body</CODE> Element
</H3>
<P>
The "body" element contains information that is related to the temporal and
linking behavior of the document. It implicitly defines a "seq" element (defined
in Section 4.2.2, see Section 4.2.4 for a definition of the temporal semantics
of the "body" element).
<P>
<STRONG>Element Attributes </STRONG>
<P>
The "body" element can have the following attribute:
<DL>
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A>
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
The "body" element can contain the following children:
<DL>
  <DT>
    a
  <DD>
    Defined in <A HREF="#a">Section 4.5.1</A>
  <DT>
    animation
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    audio
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    img
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    par
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    ref
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    seq
  <DD>
    Defined in <A HREF="#seq">Section 4.2.2</A>
  <DT>
    switch
  <DD>
    Defined in <A HREF="#switch">Section 4.3</A>
  <DT>
    text
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    textstream
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    video
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
</DL>
<H3>
  <A name="synchronization-elements">4.2</A> Synchronization Elements
</H3>
<H4>
  <A name="par">4.2.1</A> The <TT>par</TT> Element
</H4>
<P>
The children of a par element can overlap in time. The textual order of
appearance of children in a par has no significance for the timing of their
presentation.
<P>
<STRONG>Element Attributes</STRONG>
<P>
The "par" element can have the following attributes:
<DL>
  <DT>
    abstract
  <DD>
    A brief description of the content contained in the element.
  <DT>
    author
  <DD>
    The name of the author of the content contained in the element.
  <DT>
    begin
  <DD>
    This attribute specifies the time for the explicit begin of an element. See
    <A HREF="#SMIL_Time_Model">Section 4.2.4</A> for a definition of its semantics.
    <BR>
    The attribute can contain the following two types of values:
    <DL>
      <DT>
	delay-value
      <DD>
	A delay value is a clock-value measuring presentation time. Presentation
	time advances at the speed of the presentation. It behaves like the timecode
	shown on a counter of a tape-deck. It can be stopped, decreased or increased
	either by user actions, or by the player itself.<BR>
	The semantics of a delay value depend on the element's first ancestor that
	is a synchronization element (i.e. ancestors that are "a" or "switch" elements
	are ignored):
	<UL>
	  <LI>
	    If this ancestor is a "par" element, the value defines a delay from the effective
	    begin of that element (see Figure 4.1).
	  <LI>
	    If this ancestor is a "seq" element (defined in <A HREF="#seq">Section
	    4.2.2</A>), the value defines a delay from the effective end of the first
	    lexical predecessor that is a synchronization element (see Figure 4.2).
	</UL>
      <DT>
	event-value
      <DD>
	The element begins when a certain event occurs (see Figure 4.3). Its value
	is an element-event (see Definition below). <BR>
	The element generating the event must be "in scope". The set of "in scope"
	elements S is determined as follows:
	<OL>
	  <LI>
	    Take all children from the element's first ancestor that is a synchronization
	    element and add them to S.
	  <LI>
	    Remove all "a" and "switch" elements from S. Add the children of all "a"
	    elements to S, unless they are "switch" elements.
	</OL>
	<P>
	The resulting set S is the set of "in scope" elements.
    </DL>
</DL>
<P>
  <HR>
<PRE>&lt;par&gt;
  &lt;audio id="a" begin="6s" src="audio" /&gt;
&lt;/par&gt;
</PRE>
<P>
<IMG WIDTH="143" HEIGHT="61" alt="delay in 'par' element" SRC="par-offset.gif">
<A HREF="ld#longdesc-par-delay">D</A>
<P>
  <HR>
<I>Figure 4.1: Using a delay value within a "par" element</I>
  <HR>
<PRE>&lt;seq&gt;
  &lt;audio src="audio1" /&gt;
  &lt;audio begin="5s" src="audio2" /&gt;
&lt;/seq&gt;
</PRE>
<P>
<IMG WIDTH="187" HEIGHT="40" alt="delay in 'seq' element" SRC="seq-offset.gif">
<A HREF="ld#longdesc-seq-delay">D</A>
  <HR>
<I>Figure 4.2: Using a delay value within a "seq" element</I>
  <HR>
<PRE>&lt;par&gt;
  &lt;audio id="a" begin="6s" ... /&gt;
  &lt;img  begin="id(a)(4s)" ... /&gt;
&lt;/par&gt;
</PRE>
<P>
<IMG WIDTH="127" HEIGHT="98" alt="element with event value" SRC="element-event.gif">
<A HREF="ld#longdesc-event-value">D</A>
<P>
  <HR>
<I>Figure 4.3: Synchronization attribute with element event value</I>
<DL>
  <DT>
    copyright
  <DD>
    The copyright notice of the content contained in the element.
  <DT>
    dur
  <DD>
    This attribute specifies the explicit duration of an element. See
    <A HREF="#SMIL_Time_Model">Section 4.2.4</A> for a definition of its semantics.
    The attribute value can be a clock value, or the string "indefinite".
  <DT>
    end
  <DD>
    This attribute specifies the explicit end of an element. See
    <A HREF="#SMIL_Time_Model">Section 4.2.4</A> for a definition of its semantics.
    The attribute can contain the same types of attribute values as the "begin"
    attribute.
  <DT>
    <A NAME="endsync">endsync</A>
  <DD>
    For a definition of the semantics of this attribute, see
    <A HREF="#SMIL_Time_Model">Section 4.2.4</A>. The attribute can have the
    following values:
    <UL>
      <LI>
	first<FONT COLOR="Red"><BR>
	</FONT>For a definition of the semantics of this value, see
	<A HREF="#SMIL_Time_Model">Section 4.2.4</A>.
      <LI>
	id-ref<BR>
	This attribute value has the following syntax:<BR>
	<BR>
	<CODE>id-ref ::= "id(" id-value ")"</CODE><BR>
	where "id-value" must be a legal XML identifier.<FONT COLOR="Red"><BR>
	</FONT>For a definition of the semantics of this value, see
	<A HREF="#SMIL_Time_Model">Section 4.2.4</A>.
      <LI>
	last<FONT COLOR="Red"><BR>
	</FONT>For a definition of the semantics of this value, see
	<A HREF="#SMIL_Time_Model">Section 4.2.4</A>.
    </UL>
    <P>
    The default value of "endsync" is "last".
</DL>
<DL>
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A>
  <DT>
    region
  <DD>
    This attribute specifies an abstract rendering surface (either visual or
    acoustic) defined within the layout section of the document. Its value must
    be an XML identifier. If no rendering surface with this id is defined in
    the layout section, the values of the formatting properties of this element
    are determined by the default layout.<BR>
    The "region" attribute on "par" elements cannot be used by the basic layout
    language for SMIL defined in this specification. It is added for completeness,
    since it may be required by other layout languages.
  <DT>
    <A NAME="repeat">repeat</A>
  <DD>
    For a definition of the semantics of this attribute, see
    <A HREF="#SMIL_Time_Model">Section 4.2.4</A>. The attribute value can be
    an integer, or the string "indefinite". The default value is 1.
  <DT>
    <A NAME="bitrate">system-bitrate</A>
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-captions
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-<A NAME="language">language</A>
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-overdub-or-caption
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-required
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-<A NAME="screen">screen</A>-size
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    <A NAME="screen-depth">system-screen-depth</A>
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    <A NAME="title">title</A>
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A><BR>
    It &nbsp;is strongly recommended that all "par" elements have a "title" attribute
    with a meaningful description. Authoring tools should ensure that no element
    can be introduced into a SMIL document without this attribute.
</DL>
<P>
<STRONG>Note on Synchronization between Children</STRONG>
<P>
The accuracy of synchronization between the children in a parallel group
is implementation-dependent. Take the example of synchronization in case
of playback delays, i.e. the behavior when<FONT COLOR="Green"> </FONT>the
"par" element contains two or more continuous media types such as audio or
video, and one of them experiences a delay. <BR>
A player can show the following synchronization behaviors:
<DL>
  <DT>
    hard synchronization
  <DD>
    The player synchronizes the children in the "par" element to a common clock
    (see Figure 4.4 a)).
  <DT>
    soft synchronization
  <DD>
    Each child of the "par" element has its own clock, which runs independently
    of the clocks of other children in the "par" element (see Figure 4.4 b)).
</DL>
<P>
  <HR>
<P>
<IMG WIDTH="126" HEIGHT="115" alt="hard synchronization" SRC="sync-hard.gif">
<A HREF="ld#longdesc-sync-hard">D</A>
<P>
<I>a) hard synchronization: Delay in video: Either the audio is stopped,
or some video frames are dropped. The exact behavior is
implementation-dependent</I>
<P>
<IMG WIDTH="114" HEIGHT="50" alt="soft synchronization" SRC="sync-soft.gif">
<A HREF="ld#longdesc-sync-soft">D</A>
<P>
<I>b) soft synchronization</I>
<P>
  <HR>
<P>
<I>Figure 4.4: Effect of a delay on playout schedule for players using different
synchronization policies</I>
<P>
<STRONG>Attribute Values</STRONG>
<DL>
  <DT>
    clock value
  <DD>
    Clock values have the following syntax:
  <DD>
    <PRE>Clock-val             ::= Full-clock-val | Partial-clock-val | Timecount-val
Full-clock-val        ::= Hours ":" Minutes ":" Seconds ("." Fraction)?
Partial-clock-val     ::= Minutes ":" Seconds ("." Fraction)?
Timecount-val         ::= Timecount ("." Fraction)?
                          ("h" | "min" | "s" | "ms")? ; default is "s"
Hours                 ::= 2DIGIT; any positive number
Minutes               ::= 2DIGIT; range from 00 to 59
Seconds               ::= 2DIGIT; range from 00 to 59
Fraction	      ::= DIGIT+
Timecount             ::= DIGIT+
2DIGIT		      ::= DIGIT DIGIT
DIGIT		      ::= [0-9]

</PRE>
    <P>
    The following are examples of legal clock values:
    <UL>
      <LI>
	Full clock value: 02:30:03 = 2 hours, 30 minutes and 3 seconds
      <LI>
	Partial clock value: 02:33 = 2 minutes and 33 seconds
      <LI>
	Timecount values:<BR>
	3h = 3 hours<BR>
	45min = 45 minutes<BR>
	30s = 30 seconds<BR>
	5ms = 5 milliseconds
    </UL>
    <P>
    <BR>
    A fraction x with n digits represents the following value: <BR>
    <BR>
    x * 1/10**n<BR>
    <BR>
    Examples:<BR>
    <BR>
    00.5s = 5 * 1/10 seconds = 500 milliseconds<BR>
    00:00.005 = 5 * 1/1000 seconds = 5 milliseconds
</DL>
<DL>
  <DT>
    element-event value
  <DD>
    An <I>element event</I> value specifies a particular event in a synchronization
    element.<BR>
    An element event has the following syntax:<BR>
  <DD>
    <PRE>Element-event     ::= "id(" Event-source ")(" Event ")"
Event-source      ::= Id-value
Event             ::= "begin" | Clock-val | "end"<BR>
</PRE>
  <DD>
    The following events are defined:
    <DL>
      <DT>
	begin
      <DD>
	This event is generated at an element's effective begin. <BR>
	Example use: <CODE>begin="id(x)(begin)"</CODE>
      <DT>
	clock-val
      <DD>
	This event is generated when a clock associated with an element reaches a
	particular value. This clock starts at 0 at the element's effective begin.
	For "par" and "seq" elements, the clock gives the presentation time elapsed
	since the effective begin of the element. For media object elements, the
	semantics are implementation-dependent. The clock may either give presentation
	time elapsed since the effective begin, or it may give the media time of
	the object. The latter may differ from the presentation time that elapsed
	since the object's display was started e.g. due to rendering or network delays,
	and is the recommended approach.<BR>
	It is an error to use a clock value that exceeds the value of the effective
	duration of the element generating the event.
	<P>
	Example use: <CODE>begin="id(x)(45s)"</CODE>
      <DT>
	end
      <DD>
	This event&nbsp;is generated at the element's effective end. <BR>
	Example use: <CODE>begin="id(x)(end)"</CODE>
    </DL>
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
The par element can contain the following children:
<DL>
  <DT>
    a
  <DD>
    Defined in <A HREF="#a">Section 4.5.1</A>
  <DT>
    animation
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    audio
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    img
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    par
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    ref
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    seq
  <DD>
    Defined in <A HREF="#seq">Section 4.2.2</A>
  <DT>
    switch
  <DD>
    Defined in <A HREF="#switch">Section 4.3</A>
  <DT>
    text
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    textstream
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    video
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
</DL>
<P>
All of these elements may appear multiple times as direct children of a par
element.
<H4>
  <A name="seq">4.2.2</A> The <TT>seq</TT> Element
</H4>
<P>
The children of a "seq" element form a temporal sequence.
<P>
<STRONG>Attributes</STRONG>
<P>
The seq element can have the following attributes:
<DL>
  <DT>
    abstract
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    author
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    begin
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    copyright
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    dur
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    end
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A>
  <DT>
    region
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A><BR>
    The region attribute on "seq" elements cannot be used by the basic layout
    language for SMIL defined in this specification. It is added for completeness,
    since it may be required by other layout languages.
  <DT>
    repeat
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    <A NAME="bitrate">system-bitrate</A>
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-captions
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-<A NAME="language">language</A>
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-overdub-or-caption
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-required
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-<A NAME="screen">screen</A>-size
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    <A NAME="screen-depth">system-screen-depth</A>
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    title
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A><BR>
    It &nbsp;is strongly recommended that all "seq" elements have a "title" attribute
    with a meaningful description. Authoring tools should ensure that no element
    can be introduced into a SMIL document without this attribute.
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
The seq element can contain the following children:
<DL>
  <DT>
    a
  <DD>
    Defined in <A HREF="#a">Section 4.5.1</A>
  <DT>
    animation
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    audio
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    img
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    par
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    ref
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    seq
  <DD>
    Defined in <A HREF="#seq">Section 4.2.2</A>
  <DT>
    switch
  <DD>
    Defined in <A HREF="#switch">Section 4.3</A>
  <DT>
    text
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    textstream
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    video
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
</DL>
<H4>
  <A name="media-object">4.2.3</A> Media Object Elements: The <TT>ref</TT>,
  <CODE>animation</CODE>, <TT>audio,img</TT>, <TT>video,text</TT> and
  <CODE>textstream</CODE> elements
</H4>
<P>
The media object elements allow the inclusion of media objects into a SMIL
presentation. Media objects are included by reference (using a URI).
<P>
There are two types of media objects: media objects with an intrinsic duration
(e.g. video, audio) (also called "continuous media"), and media objects without
intrinsic duration (e.g. text, image) (also called "discrete media").
<P>
Anchors and links can be attached to visual media objects, i.e. media objects
rendered on a visual abstract rendering surface.
<P>
When playing back a media object, the player must not derive the exact type
of the media object from the name of the media object element. Instead, it
must rely solely on other sources about the type, such as type information
contained in the "type" attribute, or the type information communicated by
a server or the operating system.
<P>
Authors, however, should make sure that the group into which of the media
object falls (animation, audio, img, video, text or textstream) is reflected
in the element name. This is in order to increase the readability of the
SMIL document. When in doubt about the group of a media object, authors should
use the generic "ref" element.
<P>
<STRONG>Element Attributes</STRONG>
<P>
Media object elements can have the following attributes:
<DL>
  <DT>
    abstract
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    alt
  <DD>
    For user agents that cannot display a particular media-object, this attribute
    specifies alternate text.<B> </B>It &nbsp;is strongly recommended that all
    media object elements have an "alt" attribute with a meaningful description.
    Authoring tools should ensure that no element can be introduced into a SMIL
    document without this attribute.
  <DT>
    author
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    begin
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    clip-begin
  <DD>
    The clip-begin attribute specifies the beginning of a sub-clip of a continuous
    media object as offset from the start of the media object. <BR>
    Values in the clip-begin attribute have the following syntax:
    <PRE>Clip-time-value   ::= Metric "=" ( Clock-val | Smpte-val )
Metric            ::= Smpte-type | "npt"
Smpte-type        ::= "smpte" | "smpte-30-drop" | "smpte-25"
Smpte-val         ::= Hours ":" Minutes ":" Seconds
                      [ ":" Frames [ "." Subframes ]]
Hours             ::= 2DIGIT
Minutes           ::= 2DIGIT
Seconds           ::= 2DIGIT
Frames            ::= 2DIGIT
Subframes         ::= 2DIGIT
</PRE>
    <P>
    The value of this attribute consists of a metric specifier, followed by a
    time value whose syntax and semantics depend on the metric specifier. The
    following formats are allowed:
    <DL>
      <DT>
	SMPTE Timestamp
      <DD>
	SMPTE time codes <A HREF="#ref-smpte">[SMPTE]</A> can be used for frame-level
	access accuracy. The metric specifier can have the following values:
	<DL>
	  <DT>
	    smpte
	  <DT>
	    smpte-30-drop
	  <DD>
	    These values indicate the use of the "SMPTE 30 drop" format with 29.97 frames
	    per second. The "frames" field in the time value can assume the values 0
	    through 29. The difference between 30 and 29.97 frames per second is handled
	    by dropping the first two frame indices (values 00 and 01) of every minute,
	    except every tenth minute.
	  <DT>
	    smpte-25
	  <DD>
	    The "frames" field in the time specification can assume the values 0 through
	    24.
	</DL>
	<P>
	The time value has the format hours:minutes:seconds:frames.subframes. If
	the frame value is zero, it may be omitted. Subframes are measured in
	one-hundredth of a frame.<BR>
	Examples:<BR>
	<CODE>clip-begin="smpte=10:12:33:20"</CODE>
      <DT>
	Normal Play Time
      <DD>
	Normal Play Time expresses time in terms of SMIL clock values. The metric
	specifier is "npt", and the syntax of the time value is identical to the
	syntax of SMIL clock values.<BR>
	Examples:<BR>
	<CODE>clip-begin="npt=123.45s"<BR>
	clip-begin="npt=12:05:35.3</CODE>"
    </DL>
  <DT>
    clip-end
  <DD>
    The clip-end attribute specifies the end of a sub-clip of a continuous media
    object (such as audio, video or another presentation) that should be played.
    It uses the same attribute value syntax as the clip-begin
    attribute.<FONT COLOR="Red"><BR>
    </FONT>If the value of the "clip-end" attribute exceeds the duration of the
    media object, the value is ignored, and the clip end is set equal to&nbsp;the
    effective end of the media object.
  <DT>
    copyright
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    <A NAME="dur">dur</A>
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    end
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    fill
  <DD>
    For a definition of the semantics of this attribute, see Section 4.2.4. The
    attribute can have the values "remove" and "freeze".
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A>
  <DT>
    longdesc
  <DD>
    This attribute specifies a link (URI) to a long description of a media object.
    This description should supplement the short description provided using the
    alt attribute. When the media-object has associated anchors, this attribute
    should provide information about the anchor's contents.
  <DT>
    <A NAME="loc">region</A>
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    <A NAME="href">src </A>
  <DD>
    The value of the src attribute is the URI of the media object.
  <DT>
    <A NAME="bitrate">system-bitrate</A>
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-captions
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-<A NAME="language">language</A>
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-overdub-or-caption
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-required
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    system-<A NAME="screen">screen</A>-size
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    <A NAME="screen-depth">system-screen-depth</A>
  <DD>
    Defined in <A HREF="#test">Section 4.4</A>
  <DT>
    title
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A><BR>
    It is strongly recommended that all media object elements have a "title"
    attribute with a meaningful description. Authoring tools should ensure that
    no element can be introduced into a SMIL document without this attribute.
  <DT>
    <A NAME="type">type</A>
  <DD>
    MIME type of the media object referenced by the "src" attribute.
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
Media Object Elements can contain the following element:
<DL>
  <DT>
    anchor
  <DD>
    Defined in <A HREF="#anchor">Section 4.5.2</A>
</DL>
<H4>
  <A NAME="SMIL_Time_Model">4.2.4 SMIL Time Model</A>
</H4>
<H5>
  <A NAME="Time_Model_Variables">4.2.4.1 Time Model Values</A>
</H5>
<P>
In the following discussion, the term "element" refers to synchronization
elements only.
<P>
For each element we define the implicit, explicit, desired, and effective
begin, duration, and end.
<P>
The effective begin/duration/end specify what the reader of the document
will perceive.
<P>
The implicit, explicit, and desired values are auxiliary values used to define
the effective values.
<P>
The rules for calculating each of these values for the elements defined in
SMIL 1.0 are described in the next section.
<OL>
  <LI>
    Each element in SMIL has an <EM>implicit begin</EM>.
  <LI>
    Each element can be assigned an <EM>explicit begin</EM> by adding a "begin"
    attribute to the element:
    <BLOCKQUOTE>
      <CODE>begin = "<I>value of explicit-begin</I></CODE>"
    </BLOCKQUOTE>
    <P>
    It is an error if the explicit begin is earlier than the implicit begin of
    the element.
  <LI>
    Each element in SMIL has an <EM>implicit end</EM>.
  <LI>
    Each element can be assigned an <EM>explicit end</EM> by adding an "end"
    attribute to the element:
    <BLOCKQUOTE>
      <CODE>end = "<I>value of explicit-end</I>"</CODE>
    </BLOCKQUOTE>
  <LI>
    The <EM>implicit duration</EM> of an element is the difference between the
    implicit end and the implicit begin.
  <LI>
    Each element in SMIL can be assigned an <EM>explicit duration</EM> by adding
    a "dur" attribute to the element:
    <BLOCKQUOTE>
      <CODE>dur = "<I>value of explicit-duration</I>"</CODE>
    </BLOCKQUOTE>
  <LI>
    The <EM>desired begin</EM> of an element is equal to the explicit begin if
    one is given, otherwise the desired begin is equal to the implicit begin.
  <LI>
    Each element has a <EM>desired end</EM>.
  <LI>
    The <EM>desired duration</EM> of an element is the difference between the
    desired end and the desired begin.
  <LI>
    Each element has an <EM>effective begin</EM>.
  <LI>
    Each element has an <EM>effective end</EM>. (Note: the effective end of a
    child element can never be later than the effective end of its parent.)
  <LI>
    The <EM>effective duration</EM> of an element is the difference between the
    effective end and the effective begin.
</OL>
<H5>
  <A NAME="Determining_Values">4.2.4.2 Determining Time Model Values for SMIL
  1.0 Elements</A>
</H5>
<P>
This section defines how time model values are calculated for the synchronization
elements of SMIL 1.0 in cases that are not covered by the rules in
<A HREF="#Time_Model_Variables">Section 4.2.4.1</A>.
<H4>
  Determining the <EM>implicit begin</EM> of an element
</H4>
<UL>
  <LI>
    The implicit begin of the first child of the "body" element is when the document
    starts playing. When this is falls outside the scope of this document.
  <LI>
    The implicit begin of a child of a "par" element is equal to the effective
    begin of the "par" element.
  <LI>
    The implicit begin of the first child of a "seq" element is equal to the
    effective begin of the "seq" element.
  <LI>
    The implicit begin of any other child of a "seq" element is equal to the
    desired end time of the previous child of the "seq" element.
</UL>
<H4>
  Determining the <EM>implicit end</EM> of an element
</H4>
<P>
The first description that matches the element is the one that is to be used:
<UL>
  <LI>
    An element with a "repeat" attribute with value "indefinite" has an implicit
    end immediately after its effective begin.
  <LI>
    An element with a "repeat" attribute with a value other than "indefinite"
    has an implicit end equal to the implicit end of a seq element with the stated
    number of copies of the element without "repeat" attribute as children.
  <LI>
    A media object element referring to a continuous media object has an implicit
    end equal to the sum of the effective begin of the element and the intrinsic
    duration of the media object.
  <LI>
    A media object element referring to a discrete media object such as text
    or image has an implicit end immediately after its effective begin.
  <LI>
    A "seq" element has an implicit end equal to the desired end of its last
    child.
  <LI>
    A "par" element has an implicit end that depends on the value of the "endsync"
    attribute. The implicit end is equal to the sum of the effective begin of
    the "par" element and the implicit duration which is derived as follows:
    <UL>
      <LI>
	If the value of the "endsync" attribute is "last", or if the "endsync" attribute
	is missing, the implicit duration of the "par" element is the maximum of
	the desired durations of its children.
      <LI>
	If the value of the "endsync" attribute is "first", the implicit duration
	of the "par" element is the minimum of the desired durations of its children.
      <LI>
	If the value of the "endsync" attribute is an id-ref, the implicit duration
	of the "par" element is equal to the desired duration of the child referenced
	by the "id-ref".
    </UL>
</UL>
<H4>
  Determining the <EM>desired end</EM> of an element
</H4>
<UL>
  <LI>
    If the element has both an explicit duration and an explicit end, the desired
    end is the minimum of:
    <UL>
      <LI>
	the sum of the desired begin and the explicit duration; and
      <LI>
	the explicit end.
    </UL>
  <LI>
    If the element has an explicit duration but no explicit end, the desired
    end is the sum of the desired begin and the explicit duration.
  <LI>
    If the element has an explicit end but no explicit duration, the desired
    end is equal to the explicit end
  <LI>
    Otherwise, the desired end is equal to the implicit end.
</UL>
<P>
<STRONG>Determining the <EM>desired begin</EM> of an element</STRONG>
<P>
The desired begin of an element is determined by using rule 7 in
<A HREF="#Time_Model_Variables">Section 4.2.4.1</A>.
<H4>
  Determining the <EM>effective begin</EM> of an element
</H4>
<P>
The <EM>effective begin</EM> of an element is equal to the desired begin
of the element, unless the effective end of the parent element is earlier
than this time, in which case the element is not shown at all.
<H4>
  Determining the <EM>effective end</EM> of an element
</H4>
<UL>
  <LI>
    The effective end of the last child of the body element is player-dependent.
    The effective end is at least as late as the desired end, but whether it
    is any later is implementation-dependent.
  <LI>
    The effective end of the child of a "par" element can be derived as follows:
    <UL>
      <LI>
	If the child has a "fill" attribute, and the value of the "fill" attribute
	is "freeze", the effective end of the child element is equal to the effective
	end of the parent.<FONT COLOR="Red"><BR>
	</FONT>The last state of the element is retained on the screen until the
	effective end of the element.
      <LI>
	If the child has a "fill" attribute, and the value of the "fill" attribute
	is "remove", the effective end of the child element is the minimum of the
	effective end of the parent and the desired end of the child element.
      <LI>
	If the child element has no "fill" attribute, the effective end of the child
	depends on whether or not the child has an explicit duration or end.
	<UL>
	  <LI>
	    If the child has an explicit duration or end, the effective end is determined
	    as if the element had a "fill" attribute with value "remove".
	  <LI>
	    If the child has neither an explicit duration nor an explicit end, the effective
	    end is determined as if the element had a "fill" attribute with value "freeze".
	</UL>
    </UL>
  <LI>
    The effective end of the last child of a "seq" element is derived in the
    same way as the effective end of a child of a "par" element.
  <LI>
    The effective end of any other child of a "seq" element can be derived as
    follows:
    <UL>
      <LI>
	If the child has a "fill" attribute, and the value of the "fill" attribute
	is "freeze", the effective end of the child element is equal to the effective
	begin of the next element
      <LI>
	If the child has a "fill" attribute, and the value of the "fill" attribute
	is "remove", the effective end of the child element is the minimum of the
	effective begin of the next element and the desired end of the next child
	element.
      <LI>
	If the child element has no "fill" attribute, the effective end of the child
	depends on whether or not the child has an explicit duration or end.
	<UL>
	  <LI>
	    If the child has an explicit duration or end, the effective end is determined
	    as if the element had a fill attribute with value "remove".
	  <LI>
	    If the child has neither an explicit duration nor an explicit end, the effective
	    end is determined as if the element had a fill attribute with value "freeze".
	</UL>
    </UL>
</UL>
<H3>
  <A NAME="switch">4.3</A> The <TT>switch</TT> Element
</H3>
<P>
The switch element allows an author to specify a set of alternative elements
from which only one acceptable element should be chosen. An element is acceptable
if the element is a SMIL 1.0 element, the media-type can be decoded, and
all of the test-attributes (see <A HREF="#test">Section 4.4</A>) of the element
evaluate to "true".
<P>
An element is selected as follows: the player evaluates the elements in the
order in which they occur in the switch element. The first acceptable element
is selected at the exclusion of all other elements within the switch.
<P>
Thus, authors should order the alternatives from the most desirable to the
least desirable. Furthermore, authors should place a relatively fail-safe
alternative as the last item in the &lt;switch&gt; so that at least one item
within the switch is chosen (unless this is explicitly not desired).
Implementations should NOT arbitrarily pick an object within a
&lt;switch&gt; when test-attributes for all fail.
<P>
Note that http URIs provide for content-negotiation, which may be an alternative
to using the "switch" element in some cases.
<P>
<STRONG>Attributes</STRONG>
<P>
The switch element can have the following attributes:
<DL>
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A>
  <DT>
    title
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A><BR>
    It &nbsp;is strongly recommended that all switch elements have a "title"
    attribute with a meaningful description Authoring tools should ensure that
    no element can be introduced into a SMIL document without this attribute.
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
If the "switch" element is used as a direct or indirect child of a "body"
element, it can contain the following children:
<DL>
  <DT>
    a
  <DD>
    Defined in <A HREF="#a">Section 4.5.1</A>
  <DT>
    animation
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    audio
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    img
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    par
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    ref
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    seq
  <DD>
    Defined in <A HREF="#seq">Section 4.2.2</A>
  <DT>
    switch
  <DD>
    Defined in <A HREF="#switch">Section 4.3</A>
  <DT>
    text
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    textstream
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    video
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
</DL>
<P>
All of these elements may appear multiple times as children of a "switch"
element.
<P>
If the "switch" element is used within a "head" element, it can contain the
following child:
<DL>
  <DT>
    layout
  <DD>
    Defined in <A HREF="#layout">Section 3.2</A><BR>
    Multiple layout elements may occur within the switch element.
</DL>
<H3>
  <A NAME="test">4.4 </A>Test <STRONG>Attributes</STRONG>
</H3>
<P>
This specification defines a list of test attributes that can be added to
any synchronization element, and that test system capabilities and settings.
Conceptually, these attributes represent boolean tests. When one of the
test attributes specified for an element evaluates to "false", the element
carrying this attribute is ignored.
<P>
Within the list below, the concept of "user preference" may show up. User
preferences are usually set by the playback engine using a preferences dialog
box, but this specification does not place any restrictions on how such
preferences are communicated from the user to the SMIL player.
<P>
The following test attributes are defined in SMIL 1.0:
<DL>
  <DT>
    <A NAME="bitrate">system-bitrate</A>
  <DD>
    This attribute specifies the approximate bandwidth, in bits per second available
    to the system. The measurement of bandwidth is application specific, meaning
    that applications may use sophisticated measurement of end-to-end connectivity,
    or a simple static setting controlled by the user. In the latter case, this
    could for instance be used to make a choice based on the users connection
    to the network. Typical values for modem users would be 14400, 28800, 56000
    bit/s etc. Evaluates to "true" if the available system bitrate is equal to
    or greater than the given value. Evaluates to "false" if the available system
    bitrate is less than the given value. <BR>
    The attribute can assume any integer value greater than 0. If the value exceeds
    an implementation-defined maximum bandwidth value, the attribute always evaluates
    to "false".
  <DT>
    system-captions
  <DD>
    This attribute allows authors to distinguish between a redundant text equivalent
    of the audio portion of the presentation (intended for a audiences such as
    those with hearing disabilities or those learning to read who want or need
    this information) and text intended for a wide audience. The attribute can
    has the value "on" if the user has indicated a desire to see closed-captioning
    information, and it has the value "off" if the user has indicated that they
    don't wish to see such information. Evaluates to "true" if the value is "on",
    and evaluates to "false" if the value is "off".
  <DT>
    system-<A NAME="language">language</A>
  <DD>
    The attribute value is a comma-separated list of language names as defined
    in [RFC1766].
    <P>
    Evaluates to "true" if one of the languages indicated by user preferences
    exactly equals one of the languages given in the value of this parameter,
    or if one of the languages indicated by
    <P>
    user preferences exactly equals a prefix of one of the languages given in
    the value of this parameter such that the first tag character following the
    prefix is "-".
    <P>
    Evaluates to "false" otherwise.
    <P>
    Note: This use of a prefix matching rule does not imply that language tags
    are assigned to languages in such a way that it is always true that if a
    user understands a language with a certain tag, then this user will also
    understand all languages with tags for which this tag is a prefix.
    <P>
    The prefix rule simply allows the use of prefix tags if this is the case.
    <P>
    Implementation note: When making the choice of linguistic preference available
    to the user, implementors should take into account the fact that users are
    not familiar with the details of language matching as described above, and
    should provide appropriate guidance. As an example, users may assume that
    on selecting "en-gb", they will be served any kind of English document if
    British English is not available. The user interface for setting user preferences
    should guide the user to add "en" to get the best matching behavior.
    <P>
    Multiple languages MAY be listed for content that is intended for multiple
    audiences. For example, a rendition of the "Treaty of Waitangi", presented
    simultaneously in the original Maori and English versions, would call for:
    <P>
    <PRE>            &lt;audio src="foo.rm" system-language="mi, en"/&gt;
</PRE>
    <P>
    However, just because multiple languages are present within the object on
    which the system-language test attribute is placed, this does not mean that
    it is intended for multiple linguistic audiences. An example would be a
    beginner's language primer, such as "A First Lesson in Latin," which is clearly
    intended to be used by an English-literate audience. In this case, the
    system-language test attribute should only include "en".
    <P>
    Authoring note: Authors should realize that if several alternative language
    objects are enclosed in a "switch", and none of them matches, this may lead
    to situations such as a video being shown without any audio track. It is
    thus recommended to include a "catch-all" choice at the end of such a switch
    which is acceptable in all cases.
  <DT>
    system-overdub-or-caption
  <DD>
    This attribute is a setting which determines if users prefer overdubbing
    or captioning when the option is available. The attribute can have the values
    "caption" and "overdub". Evaluates to "true" if the user preference matches
    this attribute value. Evaluates to "false" if they do not match.
  <DT>
    system-required
  <DD>
    This attribute specifies the name of an extension. Evaluates to "true" if
    the extension is supported by the implementation, otherwise, this evaluates
    to "false". In a future version of SMIL, this attribute value will be an
    XML namespace <A NAME="ref-NAMESPACES">[NAMESPACES]</A>.
  <DT>
    system-<A NAME="screen">screen</A>-size
  <DD>
    Attribute values have the following syntax:<BR>
    <CODE>screen-size-val ::= screen-height"X"screen-width</CODE><BR>
    Each of these is a pixel value, and must be an integer value greater than
    0. &nbsp;Evaluates to "true" if the SMIL playback engine is capable of displaying
    a presentation of the given size. Evaluates to "false" if the SMIL playback
    engine is only capable of displaying a smaller presentation.
  <DT>
    <A NAME="screen-depth">system-screen-depth</A>
  <DD>
    This attribute specifies the depth of the screen color palette in bits required
    for displaying the element. The value must be greater than 0. Typical values
    are <FONT COLOR="Green">1</FONT>, 8, 24 .... Evaluates to "true" if the SMIL
    playback engine is capable of displaying images or video with the given color
    depth. Evaluates to "false" if the SMIL playback engine is only capable of
    displaying images or video with a smaller color depth.
</DL>
<P>
<STRONG> Examples</STRONG>
<P>
<I>1) Choosing between content with different bitrate</I>
<P>
In a common scenario, implementations may wish to allow for selection via
a "system-bitrate" parameter on elements. The media player evaluates each
of the "choices" (elements within the switch) one at a time, looking for
an acceptable bitrate given the known characteristics of the link between
the media player and media server.
<PRE>...
&lt;par&gt;
  &lt;text .../&gt;
  &lt;switch&gt;
    &lt;par system-bitrate="40000"&gt;
    ...
    &lt;/par&gt;
    &lt;par system-bitrate="24000"&gt;
    ...
    &lt;/par&gt;
    &lt;par system-bitrate="10000"&gt;
    ........
    &lt;/par&gt;
  &lt;/switch&gt;
&lt;/par&gt;
...
</PRE>
<P>
<I>2) Choosing between audio resources with different bitrate</I>
<P>
The elements within the switch may be any combination of elements. For instance,
one could merely be specifying an alternate audio track:
<PRE>...
&lt;switch&gt;
   &lt;audio src="joe-audio-better-quality" system-bitrate="16000" /&gt;
   &lt;audio src="joe-audio" system-bitrate="8000" /&gt;
&lt;/switch&gt;
...
</PRE>
<P>
<I>3) Choosing between audio resources in different languages</I>
<P>
In the following example, an audio resource is available both in French and
in English. Based on the user's preferred language, the player can choose
one of these audio resources.
<PRE>...
&lt;switch&gt;
   &lt;audio src="joe-audio-french" system-language="fr"/&gt;
   &lt;audio src="joe-audio-english" system-language="en"/&gt;
&lt;/switch&gt;
...
</PRE>
<P>
<I>4) Choosing between content written for different screens</I>
<P>
In the following example, the presentation contains alternative parts designed
for screens with different resolutions and bit-depths. Depending on the
particular characteristics of the screen, the player can choose one of the
alternatives.
<PRE>...
&lt;par&gt;
  &lt;text .../&gt;
  &lt;switch&gt;
    &lt;par system-screen-size="1280X1024" system-screen-depth="16"&gt;
    ........
    &lt;/par&gt;
    &lt;par system-screen-size="640X480" system-screen-depth="32"&gt;
    ...
    &lt;/par&gt;
    &lt;par system-screen-size="640X480" system-screen-depth="16"&gt;
    ...
    &lt;/par&gt;
  &lt;/switch&gt;
&lt;/par&gt;
...
</PRE>
<P>
<EM>5) Distinguishing caption tracks from stock tickers</EM>
<P>
In the following example, captions are shown only if the user wants captions
on.
<PRE>...
&lt;seq&gt;
  &lt;par&gt;
    &lt;audio      src="audio.rm"/&gt;
    &lt;video      src="video.rm"/&gt;
    &lt;textstream src="stockticker.rtx"/&gt;
    &lt;textstream src="closed-caps.rtx" system-captions="on"/&gt;
  &lt;/par&gt;
&lt;/seq&gt;
...
</PRE>
<P>
<EM>6) Choosing the language of overdub and caption tracks</EM>
<P>
In the following example, a French-language movie is available with English,
German, and Dutch overdub and caption tracks. The following SMIL segment
expresses this, and switches on the alternatives that the user prefers.
<PRE>...
&lt;par&gt;
&nbsp; &lt;switch&gt;
&nbsp; &nbsp; &lt;audio src="movie-aud-en.rm" system-language="en"
                system-overdub-or-caption="overdub"/&gt;
    &lt;audio src="movie-aud-de.rm" system-language="de"
                system-overdub-or-caption="overdub"/&gt;
    &lt;audio src="movie-aud-nl.rm" system-language="nl"
                system-overdub-or-caption="overdub"/&gt;
     &lt;!-- French for everyone else --&gt;
     &lt;audio src="movie-aud-fr.rm"/&gt;
   &lt;/switch&gt;
   &lt;video src="movie-vid.rm"/&gt;
   &lt;switch&gt;
     &lt;textstream src="movie-caps-en.rtx" system-language="en"
                 system-overdub-or-caption="caption"/&gt;
     &lt;textstream src="movie-caps-de.rtx" system-language="de"
                system-overdub-or-caption="caption"/&gt;
     &lt;textstream src="movie-caps-nl.rtx" system-language="nl"
                 system-overdub-or-caption="caption"/&gt;
     &lt;!-- French captions for those that really want them --&gt;
     &lt;textstream src="movie-caps-fr.rtx" system-captions="on"/&gt;
   &lt;/switch&gt;
&lt;/par&gt;
...
</PRE>
<H3>
  <A NAME="hyperlinking">4.5</A> Hyperlinking Elements
</H3>
<P>
The link elements allows the description of navigational links between objects.
<P>
SMIL provides only for in-line link elements. Links are limited to
uni-directional single-headed links (i.e. all links have exactly one source
and one destination resource). All links in SMIL are actuated by the user.
<P>
<STRONG>Handling of Links in Embedded Documents</STRONG>
<P>
Due to its integrating nature, the presentation of a SMIL document may involve
other (non-SMIL) applications or plug-ins. For example, a SMIL browser may
use an HTML plug-in to display an embedded HTML page. Vice versa, an HTML
browser may use a SMIL plug-in to display a SMIL document embedded in an
HTML page.
<P>
In such presentations, links may be defined by documents at different levels
and conflicts may arise. In this case, the link defined by the containing
document should take precedence over the link defined by the embedded object.
Note that since this might require communication between the browser and
the plug-in, SMIL implementations may choose not to comply with this
recommendation.
<P>
If a link is defined in an embedded SMIL document, traversal of the link
affects only the embedded SMIL document.
<P>
If a link is defined in a non-SMIL document which is embedded in a SMIL document,
link traversal can only affect the presentation of the embedded document
and not the presentation of the containing SMIL document. This restriction
may be released in future versions of SMIL.
<P>
<STRONG>Addressing</STRONG>
<P>
SMIL supports name fragment identifiers and the '#' connector. This means
that SMIL supports locators as currently used in HTML (e.g. it uses locators
of the form "http://foo.com/some/path#anchor1").
<P>
<STRONG>Linking to SMIL Fragments</STRONG>
<P>
A locator that points to a SMIL document may contain a fragment part (e.g.
http://www.w3.org/test.smi#par1). The fragment part is an id value that
identifies one of the elements within the referenced SMIL document. If a
link containing a fragment part is followed, the presentation should start
as if the user had fast-forwarded the presentation represented by the destination
document to the effective begin of the element designated by the fragment.
<P>
The following special cases can occur:
<OL>
  <LI>
    The element addressed by the link has a "repeat" attribute.
    <OL>
      <LI>
	If the value of the "repeat" attribute is N, all N repetitions of the element
	are played.
      <LI>
	If the value of the "repeat" attribute is "indefinite", playback ends according
	to the rules defined for repeat value "indefinite".
    </OL>
  <LI>
    The element addressed by the link is contained within another element that
    contains a "repeat" attribute.
    <OL>
      <LI>
	If the value of the "repeat" attribute is N, playback starts at the beginning
	of the element addressed by the link, followed by N-1 repetitions of the
	element containing the "repeat" attribute.
      <LI>
	If the value of the "repeat" attribute is "indefinite", playback starts at
	the beginning of the element addressed by the link. Playback ends according
	to the rules defined for repeat value "indefinite".
    </OL>
  <LI>
    The element addressed by the link is content of a "switch" element: It is
    forbidden to link to elements that are the content of "switch" elements.
</OL>
<H4>
  <A NAME="a">4.5.1</A> The <TT>a</TT> Element
</H4>
<P>
The functionality of the "a" element is very similar to the functionality
of the "a" element in HTML 4.0 <A HREF="#ref-HTML40">[HTML40]</A> . SMIL
adds an attribute "show" that controls the temporal behavior
of the source when the link is followed. For synchronization purposes, the
"a" element is transparent, i.e. it does not influence the synchronization
of its child elements. "a" elements may not be nested. An "a" element must
have an <A HREF="#href">href</A> attribute.
<P>
<STRONG>Attributes</STRONG>
<P>
The "a" element can have the following attributes:
<DL>
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A>
  <DT>
    href
  <DD>
    This attribute contains the URI of the link's
    destination.<FONT COLOR="Red"><BR>
    </FONT>The "href" attribute is required for "a" elements.
  <DT>
    <A HREF="#show">show</A>
  <DD>
    This attribute controls the behavior of the source document containing the
    link when the link is followed. It can have one of the following values:
    <UL>
      <LI>
	"replace": The current presentation is paused at its current state and is
	replaced by the destination resource. If the player offers a history mechanism,
	the source presentation resumes from the state in which it was paused when
	the user returns to it.
      <LI>
	"new": The presentation of the destination resource starts in a new context,
	not affecting the source resource.
      <LI>
	"pause": The source presentation is paused at its current state, and the
	destination resource starts in a new context. When the display of the destination
	resource ends, the source presentation resumes from the state in which it
	was paused.
    </UL>
    <P>
    The default value of "show" is "replace".
  <DT>
    title
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A><BR>
    It &nbsp;is strongly recommended that all "a" elements have a "title" attribute
    with a meaningful description. Authoring tools should ensure that no element
    can be introduced into a SMIL document without this attribute.
</DL>
<P>
<STRONG>Element Content</STRONG>
<P>
The "a" element can contain the following children:
<DL>
  <DT>
    animation
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    audio
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    img
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    par
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    ref
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    seq
  <DD>
    Defined in <A HREF="#seq">Section 4.2.2</A>
  <DT>
    switch
  <DD>
    Defined in <A HREF="#switch">Section 4.3</A>
  <DT>
    text
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    textstream
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
  <DT>
    video
  <DD>
    Defined in <A HREF="#media-object">Section 4.2.3</A>
</DL>
<P>
<STRONG> Examples</STRONG>
<P>
<I>Example 1</I>
<P>
The link starts up the new presentation replacing the presentation that was
playing.
<PRE>
&lt;a href="http://www.cwi.nl/somewhereelse.smi"&gt;
     &lt;video src="rtsp://foo.com/graph.imf" region="l_window"/&gt;
&lt;/a&gt;
</PRE>
<P>
In the example, the second line can be replaced by a reference to any valid
subtree of an SMIL presentation.
<P>
<I>Example 2</I>
<P>
The link starts up the new presentation in addition to the presentation that
was playing.
<PRE>
&lt;a href="http://www.cwi.nl/somewhereelse.smi" show="new"&gt;
     &lt;video src="rtsp://foo.com/graph.imf" region="l_window"/&gt;
&lt;/a&gt;
</PRE>
<P>
For example, this allows a SMIL player to spawn off an HTML browser.
<P>
<I>Example 3</I>
<P>
The link starts up the new presentation and pauses the presentation that
was playing.
<PRE>
&lt;a href="http://www.cwi.nl/somewhereelse.smi" show="pause"&gt;
     &lt;video src="rtsp://foo.com/graph.imf" region="l_window"/&gt;
&lt;/a&gt;
</PRE>
<P>
<I>Example 4 </I>
<P>
The following example contains a link from an element in one presentation
A to the middle of another presentation B. This would play presentation B
starting from the effective begin of the element with id "next".
<PRE>
Presentation A:

     &lt;a href="http://www.cwi.nl/presentationB#next"&gt;
       &lt;video src="rtsp://foo.com/graph.imf"/&gt;
     &lt;/a&gt;


Presentation B (http://www.cwi.nl/presentation):

      ...
      &lt;seq&gt;
        &lt;video src="rtsp://foo.com/graph.imf"/&gt;
        &lt;par&gt;
          &lt;video src="rtsp://foo.com/timbl.rm" region="l_window"/&gt;
          &lt;video id="next" src="rtsp://foo.com/v1.rm" region="r_window"/&gt;
                 ^^^^^^^^^
          &lt;text src="rtsp://foo.com/caption1.html" region="l_2_title"/&gt;
          &lt;text src="rtsp://foo.com/caption2.rtx" region="r_2_title"/&gt;
        &lt;/par&gt;
      &lt;/seq&gt;
      ...
</PRE>
<H4>
  <A name="anchor">4.5.2 The <CODE>anchor</CODE> Element</A>
</H4>
<P>
The functionality of the "a" element is restricted in that it only allows
associating a link with a complete media object. HTML image maps have
demonstrated that it is useful to associate links with spatial subparts of
an object. The anchor element realizes similar functionality for SMIL:
<OL>
  <LI>
    The anchor element allows associating a link destination to spatial and temporal
    subparts of a media object, using the "href" attribute (in contrast, the
    "a" element only allows associating a link with a complete media object).
  <LI>
    The anchor element allows making a subpart of the media object the destination
    of a link, using the "id" attribute.
  <LI>
    The anchor element allows breaking up an object into spatial subparts, using
    the "coords" attribute.
  <LI>
    The anchor element allows breaking up an object into temporal subparts, using
    the "begin" and "end" attributes. The values of the begin and end attributes
    are relative to the beginning of the media object.
</OL>
<P>
<STRONG>Attributes</STRONG>
<P>
The anchor element can have the following attributes:
<DL>
  <DT>
    begin
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    <A NAME="coords">coords</A>
  <DD>
    The value of this attribute specifies a rectangle within the display area
    of a visual media object. Syntax and semantics of this attribute are similar
    to the coords attribute in HTML image maps, when the link is associated with
    a rectangular shape. The rectangle is specified by four length values: The
    first two values specify the coordinates of the upper left corner of the
    rectangle.The second two values specify the coordinates of the lower right
    corner of the rectangle. Coordinates are relative to the top left corner
    of the visual media object (see Figure 4.5). If a coordinate is specified
    as a percentage value, it is relative to the total width or height of the
    media object display area. <BR>
    An attribute with an erroneous coords value is ignored (right-x smaller or
    equal to left-x, bottom-y smaller or equal to top-y). If the rectangle defined
    by the coords attribute exceeds the area covered by the media object, exceeding
    height and width are clipped at the borders of the media object.<BR>
    Values of the coords attribute have the following syntax:<BR>
    <PRE>coords-value ::= left-x "," top-y "," right-x "," bottom-y

</PRE>
</DL>
<P>
  <HR>
<IMG WIDTH="266" HEIGHT="228" alt="semantics of coords attribute" SRC="coords.gif">
<A HREF="ld#longdesc-coords">D</A>
<P>
  <HR>
<P>
<I>Figure 4.5: Semantics of "coords" attribute</I>
<DL>
  <DT>
    end
  <DD>
    Defined in <A HREF="#par">Section 4.2.1</A>
  <DT>
    id
  <DD>
    Defined in <A HREF="#smil">Section 2</A>
  <DT>
    show
  <DD>
    Defined in <A HREF="#a">Section 4.5.1</A>
  <DT>
    skip-content
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A>
  <DT>
    title
  <DD>
    Defined in <A HREF="#region">Section 3.3.1</A><BR>
    It &nbsp;is strongly recommended that all anchor elements have a "title"
    attribute with a meaningful description. Authoring tools should ensure that
    no element can be introduced into a SMIL document without this attribute.
</DL>
<P>
<STRONG>Examples</STRONG>
<P>
<I>1) Associating&nbsp;links with spatial subparts</I>
<P>
In the following example, the screenspace taken up by a video clip is split
into two sections. A different link is associated with each of these sections.
<PRE>&lt;video src="http://www.w3.org/CoolStuff"&gt;
&nbsp; &lt;anchor href="http://www.w3.org/AudioVideo" coords="0%,0%,50%,50%"/&gt;
&nbsp; &lt;anchor href="http://www.w3.org/Style"      coords="50%,50%,100%,100%"/&gt;
&lt;/video&gt;
</PRE>
<P>
<I>2) Associating links with temporal subparts</I>
<P>
In the following example, the duration of a video clip is split into two
subintervals. A different link is associated with each of these subintervals.
<PRE>&lt;video src="http://www.w3.org/CoolStuff"&gt;
&nbsp; &lt;anchor href="http://www.w3.org/AudioVideo" begin="0s" end="5s"/&gt;
&nbsp; &lt;anchor href="http://www.w3.org/Style"      begin="5s" end="10s"/&gt;
&lt;/video&gt;
</PRE>
<P>
<I>3) Jumping to a subpart of an object</I>
<P>
The following example contains a link from an element in one presentation
A to the middle of a video object contained in another presentation B. This
would play presentation B starting from second 5 in the video (i.e. the
presentation would start as if the user had fast-forwarded the whole presentation
to the point at which the designated fragment in the "CoolStuff" video begins).
<PRE>Presentation A:

&lt;a href="http://www.cwi.nl/mm/presentationB#tim"&gt;
   &lt;video id="graph" src="rtsp://foo.com/graph.imf" region="l_window"/&gt;
&lt;/a&gt;


Presentation B:

&lt;video src="http://www.w3.org/CoolStuff"&gt;
&nbsp; &lt;anchor id="joe" begin="0s" end="5s"/&gt;
&nbsp; &lt;anchor id="tim" begin="5s" end="10s"/&gt;
&lt;/video&gt;
</PRE>
<P>
<I>4) Combining different uses of links</I>
<P>
The following example shows how the different uses of associated links can
be used in combination.
<PRE>Presentation A:

&lt;a href="http://www.cwi.nl/mm/presentationB#tim"&gt;
  &lt;video id="graph" src="rtsp://foo.com/graph.imf" region="l_window"/&gt;
&lt;/a&gt;


Presentation B:

&lt;video src="http://www.w3.org/CoolStuff"&gt;
&nbsp; &lt;anchor id="joe" begin="0s" end="5s" coords="0%,0%,50%,50%"
          href="http://www.w3.org/"/&gt;
&nbsp; &lt;anchor id="tim" begin="5s" end="10s" coords="0%,0%,50%,50%"
          href="http://www.w3.org/Tim"/&gt;
&lt;/video&gt;
</PRE>
<H2>
  <A name="smil-dtd">5 SMIL DTD</A>
</H2>
<H3>
  <A name="relation">5.1 <STRONG>Relation to XML</STRONG></A>
</H3>
<P>
A SMIL 1.0 document may optionally contain a document type declaration,
which&nbsp;names the document type definition (DTD) in use for the document.
For SMIL, the document type declaration should look as follows (the double
quotes can be replaced by single quotes):
<P>
<CODE>&lt;!DOCTYPE smil PUBLIC "-//W3C//DTD SMIL 1.0//EN"<BR>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp;"http://www.w3.org/TR/REC-smil/SMIL10.dtd"&gt;</CODE>
<P>
The XML 1.0 specification provides a way to extend the DTD using the
&lt;!DOCTYPE&gt; element, for instance to add a new set of entity definitions.
Authors must not use this feature with SMIL since many SMIL players will
not support it.
<P>
The following is illegal in SMIL:
<P>
<CODE>&lt;!DOCTYPE smil PUBLIC "-//W3C//DTD SMIL 1.0//EN"<BR>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp;"http://www.w3.org/TR/REC-smil/SMIL10.dtd" [<BR>
&lt;!ENTITY % AcmeCorpSymbols PUBLIC<BR>
&nbsp; &nbsp; &nbsp;"-//Acme Corp//ENTITIES Corporate Symbols//EN"<BR>
&nbsp; &nbsp; &nbsp;"http://www.acme.com/corp_symbols.xml"<BR>
&gt;<BR>
%AcmeCorpSymbols;<BR>
]&gt;</CODE>
<H3>
  <A NAME="dtd">5.2 DTD</A>
</H3>
<PRE>&lt;!--

    This is the XML document type definition (DTD) for SMIL 1.0.

    Date: 1998/06/15 08:56:30

    Authors:
        Jacco van Ossenbruggen &lt;jrvosse@cwi.nl&gt;
        Sjoerd Mullender       &lt;sjoerd@cwi.nl&gt;

    Further information about SMIL is available at:

          http://www.w3.org/AudioVideo/

--&gt;

&lt;!-- Generally useful entities --&gt;
&lt;!ENTITY % id-attr "id ID #IMPLIED"&gt;
&lt;!ENTITY % title-attr "title CDATA #IMPLIED"&gt;
&lt;!ENTITY % skip-attr "skip-content (true|false) 'true'"&gt;
&lt;!ENTITY % desc-attr "
        %title-attr;
        abstract        CDATA   #IMPLIED
        author          CDATA   #IMPLIED
        copyright       CDATA   #IMPLIED
"&gt;

&lt;!--=================== SMIL Document =====================================--&gt;
&lt;!--
     The root element SMIL contains all other elements.
--&gt;
&lt;!ELEMENT smil (head?,body?)&gt;
&lt;!ATTLIST smil
        %id-attr;
&gt;

&lt;!--=================== The Document Head =================================--&gt;
&lt;!ENTITY % layout-section "layout|switch"&gt;

&lt;!ENTITY % head-element "(meta*,((%layout-section;), meta*))?"&gt;

&lt;!ELEMENT head %head-element;&gt;
&lt;!ATTLIST head %id-attr;&gt;


&lt;!--=================== Layout Element ====================================--&gt;
&lt;!--
     Layout contains the region and root-layout elements defined by
     smil-basic-layout or other elements defined an external layout
     mechanism.
--&gt;
&lt;!ELEMENT layout ANY&gt;
&lt;!ATTLIST layout
        %id-attr;
        type CDATA      "text/smil-basic-layout"
&gt;


&lt;!--=================== Region Element ===================================--&gt;
&lt;!ENTITY % viewport-attrs "
        height              CDATA    #IMPLIED
        width               CDATA    #IMPLIED
        background-color    CDATA    #IMPLIED
"&gt;

&lt;!ELEMENT region EMPTY&gt;
&lt;!ATTLIST region
        %id-attr;
        %title-attr;
        %viewport-attrs;
        left                CDATA    "0"
        top                 CDATA    "0"
        z-index             CDATA    "0"
        fit                 (hidden|fill|meet|scroll|slice)    "hidden"
        %skip-attr;
&gt;

&lt;!--=================== Root-layout Element ================================--&gt;
&lt;!ELEMENT root-layout EMPTY&gt;
&lt;!ATTLIST root-layout
        %id-attr;
        %title-attr;
        %viewport-attrs;
        %skip-attr;
&gt;


&lt;!--=================== Meta Element=======================================--&gt;
&lt;!ELEMENT meta EMPTY&gt;
&lt;!ATTLIST meta
        name    NMTOKEN #REQUIRED
        content CDATA   #REQUIRED
        %skip-attr;
&gt;

&lt;!--=================== The Document Body =================================--&gt;
&lt;!ENTITY % media-object "audio|video|text|img|animation|textstream|ref"&gt;
&lt;!ENTITY % schedule "par|seq|(%media-object;)"&gt;
&lt;!ENTITY % inline-link "a"&gt;
&lt;!ENTITY % assoc-link "anchor"&gt;
&lt;!ENTITY % link "%inline-link;"&gt;
&lt;!ENTITY % container-content "(%schedule;)|switch|(%link;)"&gt;
&lt;!ENTITY % body-content "(%container-content;)"&gt;

&lt;!ELEMENT body (%body-content;)*&gt;
&lt;!ATTLIST body %id-attr;&gt;

&lt;!--=================== Synchronization Attributes ========================--&gt;
&lt;!ENTITY % sync-attributes "
        begin   CDATA   #IMPLIED
        end     CDATA   #IMPLIED
"&gt;

&lt;!--=================== Switch Parameter Attributes =======================--&gt;
&lt;!ENTITY % system-attribute "
        system-bitrate             CDATA                #IMPLIED
        system-language            CDATA                #IMPLIED
        system-required            NMTOKEN              #IMPLIED
        system-screen-size         CDATA                #IMPLIED
        system-screen-depth        CDATA                #IMPLIED
        system-captions            (on|off)             #IMPLIED
        system-overdub-or-caption  (caption|overdub)    #IMPLIED
"&gt;

&lt;!--=================== Fill Attribute ====================================--&gt;
&lt;!ENTITY % fill-attribute "
        fill    (remove|freeze)    'remove'
"&gt;


&lt;!--=================== The Parallel Element ==============================--&gt;
&lt;!ENTITY % par-content "%container-content;"&gt;
&lt;!ELEMENT par    (%par-content;)*&gt;
&lt;!ATTLIST par
        %id-attr;
        %desc-attr;
        endsync CDATA           "last"
        dur     CDATA           #IMPLIED
        repeat  CDATA           "1"
        region  IDREF           #IMPLIED
        %sync-attributes;
        %system-attribute;
&gt;

&lt;!--=================== The Sequential Element ============================--&gt;
&lt;!ENTITY % seq-content "%container-content;"&gt;
&lt;!ELEMENT seq    (%seq-content;)*&gt;
&lt;!ATTLIST seq
        %id-attr;
        %desc-attr;
        dur     CDATA           #IMPLIED
        repeat  CDATA           "1"
        region  IDREF           #IMPLIED
        %sync-attributes;
        %system-attribute;
&gt;

&lt;!--=================== The Switch Element ================================--&gt;
&lt;!-- In the head, a switch may contain only layout elements,
     in the body, only container elements. However, this
     constraint cannot be expressed in the DTD (?), so
     we allow both:
--&gt;
&lt;!ENTITY % switch-content "layout|(%container-content;)"&gt;
&lt;!ELEMENT switch (%switch-content;)*&gt;
&lt;!ATTLIST switch
        %id-attr;
        %title-attr;
&gt;

&lt;!--=================== Media Object Elements =============================--&gt;
&lt;!-- SMIL only defines the structure. The real media data is
     referenced by the src attribute of the media objects.
--&gt;

&lt;!-- Furthermore, they have the following attributes as defined
     in the SMIL specification:
--&gt;
&lt;!ENTITY % mo-attributes "
        %id-attr;
        %desc-attr;
        region     IDREF           #IMPLIED
        alt        CDATA           #IMPLIED
        longdesc   CDATA           #IMPLIED
        src        CDATA           #IMPLIED
        type       CDATA           #IMPLIED
        dur        CDATA           #IMPLIED
        repeat     CDATA           '1'
        %fill-attribute;
        %sync-attributes;
        %system-attribute;
"&gt;

&lt;!--
     Most info is in the attributes, media objects are empty or
     contain associated link elements:
--&gt;
&lt;!ENTITY % mo-content "(%assoc-link;)*"&gt;
&lt;!ENTITY % clip-attrs "
        clip-begin      CDATA   #IMPLIED
        clip-end        CDATA   #IMPLIED
"&gt;

&lt;!ELEMENT ref           %mo-content;&gt;
&lt;!ELEMENT audio         %mo-content;&gt;
&lt;!ELEMENT img           %mo-content;&gt;
&lt;!ELEMENT video         %mo-content;&gt;
&lt;!ELEMENT text          %mo-content;&gt;
&lt;!ELEMENT textstream    %mo-content;&gt;
&lt;!ELEMENT animation     %mo-content;&gt;

&lt;!ATTLIST ref           %mo-attributes; %clip-attrs;&gt;
&lt;!ATTLIST audio         %mo-attributes; %clip-attrs;&gt;
&lt;!ATTLIST video         %mo-attributes; %clip-attrs;&gt;
&lt;!ATTLIST animation     %mo-attributes; %clip-attrs;&gt;
&lt;!ATTLIST textstream    %mo-attributes; %clip-attrs;&gt;
&lt;!ATTLIST text          %mo-attributes;&gt;
&lt;!ATTLIST img           %mo-attributes;&gt;

&lt;!--=================== Link Elements =====================================--&gt;

&lt;!ENTITY % smil-link-attributes "
        %id-attr;
        %title-attr;
        href            CDATA                   #REQUIRED
        show            (replace|new|pause)     'replace'
"&gt;


&lt;!--=================== Inline Link Element ===============================--&gt;
&lt;!ELEMENT a (%schedule;|switch)*&gt;
&lt;!ATTLIST a
        %smil-link-attributes;
&gt;


&lt;!--=================== Associated Link Element ===========================--&gt;
&lt;!ELEMENT anchor EMPTY&gt;
&lt;!ATTLIST anchor
        %skip-attr;
        %smil-link-attributes;
        %sync-attributes;
        coords          CDATA                   #IMPLIED
&gt;
</PRE>
<H2>
  References
</H2>
<DL>
  <DT>
    <B><A NAME="ref-CSS2">[CSS2]</A></B>
  <DD>
    "Cascading Style Sheets, level 2", B. Bos, H. Lie, C. Lilley, I. Jacobs,
    12 May 1998.
  <DD>
    Available at
    <A HREF="http://www.w3.org/TR/REC-CSS2/">http://www.w3.org/TR/REC-CSS2/</A>.
  <DT>
    <STRONG><A name="ref-HTML40" class="normref">[HTML40]</A></STRONG>
  <DD>
    "HTML 4.0 Specification", D. Raggett, A. Le Hors, I. Jacobs, 24 April 1998.<BR>
    Available at <A href="http://www.w3.org/TR/REC-html40">
    http://www.w3.org/TR/REC-html40/</A>.
  <DT>
    <STRONG><A NAME="ref-ISO/IEC 10646">[ISO/IEC 10646]</A></STRONG>
  <DD>
    ISO (International Organization for Standardization). ISO/IEC 10646-1993
    (E). Information technology -- Universal Multiple-Octet Coded Character Set
    (UCS) -- Part 1: Architecture and Basic Multilingual Plane. [Geneva]:
    International Organization for Standardization, 1993 (plus amendments AM
    1 through AM 7).
  <DT>
    <B><A NAME="ref-NAMESPACES">[NAMESPACES]</A></B>
  <DD>
    "Namespaces in XML", T. Bray, D. Hollander, A. Layman, 27 March 1998<BR>
    W3C working draft. Available at
    <A HREF="http://www.w3.org/TR/WD-xml-names">http://www.w3.org/TR/WD-xml-names</A>.
  <DT>
    <A NAME="rec-PICS"><B>[PICS]</B></A>
  <DD>
    "PICS 1.1 Label Distribution -- Label Syntax and Communication Protocols",
    31 October 1996, T. Krauskopf, J. Miller, P. Resnick, W. Trees<BR>
    Available at
    <A HREF="http://www.w3.org/TR/REC-PICS-labels-961031">http://www.w3.org/TR/REC-PICS-labels-961031</A>
  <DT>
    <STRONG><A name="ref-RFC1738" class="normref">[RFC1738]</A></STRONG>
  <DD>
    "Uniform Resource Locators", T. Berners-Lee, L. Masinter, and M. McCahill,
    December 1994.<BR>
    Available at
    <A HREF="ftp://ftp.isi.edu/in-notes/rfc1738.txt">ftp://ftp.isi.edu/in-notes/rfc1738.txt</A>.
  <DT>
    <B><A NAME="ref-RFC1766">[RFC1766]</A></B>
  <DD>
    "Tags for the Identification of Languages", H. Alvestrand, March 1995.<BR>
    Available at <A HREF="ftp://ftp.isi.edu/in-notes/rfc1766.txt">
    ftp://ftp.isi.edu/in-notes/rfc1766.txt</A>.
  <DT>
    <STRONG><A name="ref-RFC1808" class="normref">[RFC1808]</A></STRONG>
  <DD>
    "Relative Uniform Resource Locators", R. Fielding, June 1995.<BR>
    Available at
    <A HREF="ftp://ftp.isi.edu/in-notes/rfc1808.txt">ftp://ftp.isi.edu/in-notes/rfc1808.txt</A>.
  <DT>
    <STRONG><A name="ref-RFC2045" class="normref">[RFC2045]</A></STRONG>
  <DD>
    "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet
    Message Bodies", N. Freed and N. Borenstein, November 1996.<BR>
    Available at <A HREF="ftp://ftp.isi.edu/in-notes/rfc2045.txt">
    ftp://ftp.isi.edu/in-notes/rfc2045.txt</A>. Note that this RFC obsoletes
    RFC1521, RFC1522, and RFC1590.
  <DT>
    <A NAME="ref-SMPTE"><STRONG>[SMPTE]</STRONG></A>
  <DD>
    "Time and Control Codes for 24, 25 or 30 Frame-Per-Second Motion-Picture
    Systems - RP 136-1995". &nbsp;Society of Motion Picture &amp; Television
    Engineers.
  <DT>
    <STRONG><A name="ref-URI" class="normref">[URI]</A></STRONG>
  <DD>
    "Uniform Resource Identifiers (URI): Generic Syntax and Semantics", T.
    Berners-Lee, R. Fielding, L. Masinter, 4 March 1998.<BR>
    Available at
    <A HREF="http://www.ics.uci.edu/pub/ietf/uri/draft-fielding-uri-syntax-02.txt">
    http://www.ics.uci.edu/pub/ietf/uri/draft-fielding-uri-syntax-02.txt</A>.
    This is a work in progress that is expected to update
    <A href="#ref-RFC1738">[RFC1738]</A> and
    <A href="#ref-RFC1808">[RFC1808]</A>.
  <DT>
    <STRONG><A name="ref-XML10" class="normref">[XML10]</A></STRONG>
  <DD>
    "Extensible Markup Language (XML) 1.0", T. Bray, J. Paoli, C.M. Sperberg-McQueen,
    editors, 10 February 1998.<BR>
    Available at <A href="http://www.w3.org/TR/REC-xml">
    http://www.w3.org/TR/REC-xml/</A>
</DL>
<H2>
  <A NAME="Appendix">Appendix</A>
</H2>
<H3>
  <A name="handling-extensions">Extending SMIL 1.0</A>
</H3>
<P>
<EM>(non-normative)</EM>
<P>
In the future, SMIL 1.0 may be extended by another W3C recommendation, or
by private extensions.
<P>
For these extensions, it is recommended that the following rules are obeyed:
<UL>
  <LI>
    All elements introduced in extensions must have a "skip-content" attribute
    (defined in Section 3.3.1) if it should be possible that their content is
    processed by SMIL 1.0 players.
  <LI>
    Private extensions must be introduced using the syntax of the XML namespace
    specification.
</UL>
<P>
It is recommended that SMIL 1.0 players are prepared to handle documents
that contain extension that obey these two rules.
<P>
Extensions should be handled using an XML namespace mechanism, once such
a mechanism becomes a W3C recommendation. In the rest of the section, the
syntax and semantics for XML namespaces defined in the W3C note [NAMESPACE]
will be used for demonstration purposes only.
<P>
The following cases can occur:
<OL>
  <LI>
    The document contains a namespace declaration for the SMIL 1.0 specification
    that defines an empty prefix. In this case, non-SMIL 1.0 elements and attributes
    are only allowed in a document if they are declared using an XML namespace.
    The document may not contain a document type declaration for SMIL 1.0. If
    it does, it is invalid.<BR>
    In the following example, the element "new:a" is a legal extension. The elements
    "mytags:a" and "b" are syntax errors, since they are not declared using an
    XML namespace.
    <PRE>&lt;?xml:namespace ns="http://www.acme.com/new-smil" prefix="new" ?&gt;
&lt;?xml:namespace ns="http://www.w3.org/TR/PR-smil" ?&gt;
&lt;smil&gt;
&nbsp; &lt;body&gt;
&nbsp; &nbsp; &lt;par&gt;
      &lt;new:a&gt;
      ...
      &lt;/new:a&gt;
      &lt;mytags:a ... /&gt;
        ...
      &lt;/mytags:a&gt;<BR>      &lt;b&gt;
       ...
      &lt;/b&gt;
    &lt;/par&gt;
   &lt;/body&gt;
 &lt;/smil&gt;
</PRE>
  <LI>
    The document contains no document type declaration, it contains a document
    type declaration for a SMIL version higher than 1.0, or it contains a namespace
    declaration for a SMIL specification with a version higher than 1.0. For
    a SMIL 1.0 player to be able to recognize such a namespace declaration, it
    is recommended that the URI of future SMIL versions starts with
    http://www.w3.org/TR/REC-smil, and is followed by more characters which may
    for example be a&nbsp;version number.<BR>
    In this case, a SMIL 1.0 player should assume that it is processing a SMIL
    document with a version number higher than 1.0.<BR>
    The following cases can occur:
    <DL>
      <DT>
	Unknown element
      <DD>
	Unknown elements are ignored<BR>
	An unknown element may contain content that consists of SMIL 1.0 elements.
	Whether such content is ignored or processed depends on the value of the
	"skip-content" attribute. If the attribute is set to "true", or the attribute
	is absent, the content is not processed. If it is set to "false", the content
	is processed.
      <DT>
	Content in Element that was declared "Empty"
      <DD>
	A future version of SMIL may allow content in elements that are declared
	as "empty" in SMIL 1.0.
      <DD>
	Whether this content is ignored or not depends on the value of the "skip-content"
	attribute of the formerly empty element. If the attribute is set to "true",
	the content is not processed. If it is set to "false", the content is processed.
      <DT>
	Unknown Attribute
      <DD>
	Unknown attributes are ignored.
      <DT>
	Unknown Attribute Value
      <DD>
	Attributes with unknown attribute values are ignored.
    </DL>
  <LI>
    The document contains a document type declaration for SMIL 1.0. In this case,
    it may not contain any non-SMIL 1.0 elements, even if they are declared using
    XML namespaces. This is because such extensions would render the document
    invalid.
</OL>
<H3>
  <A NAME="Using">Using SMIL 1.0 as an Extension</A>
</H3>
<P>
When the XML namespace mechanism is used to include SMIL elements and attributes
in other XML-based documents, it is recommended to use the following namespace
identifier: <CODE>http://www.w3.org/TR/REC-smil</CODE>
</BODY></HTML>