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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html lang="en-US-x-Hixie" ><head><title>2 Common infrastructure &#8212; HTML5 </title><style type="text/css">
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   <h1>HTML5</h1>
   </div><div>
   <a href="introduction.html" class="prev">1 Introduction</a> &#8211;
   <a href="spec.html#contents">Table of contents</a> &#8211;
   <a href="common-microsyntaxes.html" class="next">2.5 Common microsyntaxes</a>
  <ol class="toc"><li><a href="infrastructure.html#infrastructure"><span class="secno">2 </span>Common infrastructure</a>
  <ol><li><a href="infrastructure.html#terminology"><span class="secno">2.1 </span>Terminology</a>
    <ol><li><a href="infrastructure.html#resources"><span class="secno">2.1.1 </span>Resources</a></li><li><a href="infrastructure.html#xml"><span class="secno">2.1.2 </span>XML</a></li><li><a href="infrastructure.html#dom-trees"><span class="secno">2.1.3 </span>DOM trees</a></li><li><a href="infrastructure.html#scripting-0"><span class="secno">2.1.4 </span>Scripting</a></li><li><a href="infrastructure.html#plugins"><span class="secno">2.1.5 </span>Plugins</a></li><li><a href="infrastructure.html#character-encodings"><span class="secno">2.1.6 </span>Character encodings</a></li></ol></li><li><a href="infrastructure.html#conformance-requirements"><span class="secno">2.2 </span>Conformance requirements</a>
    <ol><li><a href="infrastructure.html#conformance-classes"><span class="secno">2.2.1 </span>Conformance classes</a></li><li><a href="infrastructure.html#dependencies"><span class="secno">2.2.2 </span>Dependencies</a></li><li><a href="infrastructure.html#extensibility"><span class="secno">2.2.3 </span>Extensibility</a></li></ol></li><li><a href="infrastructure.html#case-sensitivity-and-string-comparison"><span class="secno">2.3 </span>Case-sensitivity and string comparison</a></li><li><a href="infrastructure.html#utf-8"><span class="secno">2.4 </span>UTF-8</a></li></ol></li></ol></div>

  <h2 id="infrastructure"><span class="secno">2 </span>Common infrastructure</h2><h3 id="terminology"><span class="secno">2.1 </span>Terminology</h3><p>This specification refers to both HTML and XML attributes and IDL
  attributes, often in the same context. When it is not clear which is
  being referred to, they are referred to as <dfn title="">content
  attributes</dfn> for HTML and XML attributes, and <dfn title="">IDL
  attributes</dfn> for those defined on IDL interfaces. Similarly, the
  term "properties" is used for both JavaScript object properties and
  CSS properties. When these are ambiguous they are qualified as <dfn title="">object properties</dfn> and <dfn title="">CSS
  properties</dfn> respectively.</p><p>Generally, when the specification states that a feature applies
  to <a href="syntax.html#syntax">the HTML syntax</a> or <a href="the-xhtml-syntax.html#the-xhtml-syntax">the XHTML syntax</a>, it
  also includes the other. When a feature specifically only applies to
  one of the two languages, it is called out by explicitly stating
  that it does not apply to the other format, as in "for HTML,
  ... (this does not apply to XHTML)".</p><p>This specification uses the term <dfn title="">document</dfn> to
  refer to any use of HTML, ranging from short static documents to
  long essays or reports with rich multimedia, as well as to
  fully-fledged interactive applications.</p><p>For simplicity, terms such as <dfn title="">shown</dfn>, <dfn title="">displayed</dfn>, and <dfn title="">visible</dfn> might
  sometimes be used when referring to the way a document is rendered
  to the user. These terms are not meant to imply a visual medium;
  they must be considered to apply to other media in equivalent
  ways.</p><div class="impl">

  <p>When an algorithm B says to return to another algorithm A, it
  implies that A called B. Upon returning to A, the implementation
  must continue from where it left off in calling B.</p>

  </div><p>The term "transparent black" refers to the color with red, green,
  blue, and alpha channels all set to zero.</p><h4 id="resources"><span class="secno">2.1.1 </span>Resources</h4><p>The specification uses the term <dfn title="">supported</dfn>
  when referring to whether a user agent has an implementation capable
  of decoding the semantics of an external resource. A format or type
  is said to be <i>supported</i> if the implementation can process an
  external resource of that format or type without critical aspects of
  the resource being ignored. Whether a specific resource is
  <i>supported</i> can depend on what features of the resource's
  format are in use.</p><p class="example">For example, a PNG image would be considered to
  be in a supported format if its pixel data could be decoded and
  rendered, even if, unbeknownst to the implementation, the image also
  contained animation data.</p><p class="example">A MPEG4 video file would not be considered to be
  in a supported format if the compression format used was not
  supported, even if the implementation could determine the dimensions
  of the movie from the file's metadata.</p><p>What some specifications, in particular the HTTP and URI
  specifications, refer to as a <i>representation</i> is referred to
  in this specification as a <dfn title="">resource</dfn>. <a href="references.html#refsHTTP">[HTTP]</a> <a href="references.html#refsRFC3986">[RFC3986]</a></p><p>The term <dfn id="mime-type">MIME type</dfn> is used to refer to what is
  sometimes called an <i>Internet media type</i> in protocol
  literature. The term <i>media type</i> in this specification is used
  to refer to the type of media intended for presentation, as used by
  the CSS specifications. <a href="references.html#refsRFC2046">[RFC2046]</a> <a href="references.html#refsMQ">[MQ]</a></p><p>A string is a <dfn id="valid-mime-type">valid MIME type</dfn> if it matches the <code title="">media-type</code> rule defined in section 3.7 "Media Types"
  of RFC 2616. In particular, a <a href="#valid-mime-type">valid MIME type</a> may
  include MIME type parameters. <a href="references.html#refsHTTP">[HTTP]</a></p><p>A string is a <dfn id="valid-mime-type-with-no-parameters">valid MIME type with no parameters</dfn> if it
  matches the <code title="">media-type</code> rule defined in section
  3.7 "Media Types" of RFC 2616, but does not contain any U+003B
  SEMICOLON characters (;). In other words, if it consists only of a
  type and subtype, with no MIME Type parameters. <a href="references.html#refsHTTP">[HTTP]</a></p><p>The term <dfn id="html-mime-type">HTML MIME type</dfn> is used to refer to the <a href="#mime-type" title="MIME type">MIME types</a> <code><a href="iana.html#text-html">text/html</a></code> and
  <code><a href="iana.html#text-html-sandboxed">text/html-sandboxed</a></code>.</p><p>A resource's <dfn id="critical-subresources">critical subresources</dfn> are those that the
  resource needs to have available to be correctly processed. Which
  resources are considered critical or not is defined by the
  specification that defines the resource's format. For CSS resources,
  only <code title="">@import</code> rules introduce <a href="#critical-subresources">critical
  subresources</a>; other resources, e.g. fonts or backgrounds, are
  not.</p><p>The term <dfn id="data-protocol" title="data protocol"><code title="">data:</code>
  URL</dfn> refers to <a href="urls.html#url" title="URL">URLs</a> that use the <code title="">data:</code> scheme. <a href="references.html#refsRFC2397">[RFC2397]</a></p><h4 id="xml"><span class="secno">2.1.2 </span>XML</h4><p id="html-namespace">To ease migration from HTML to XHTML, UAs
  conforming to this specification will place elements in HTML in the
  <code>http://www.w3.org/1999/xhtml</code> namespace, at least for
  the purposes of the DOM and CSS. The term "<dfn id="html-elements">HTML
  elements</dfn>", when used in this specification, refers to any
  element in that namespace, and thus refers to both HTML and XHTML
  elements.</p><p>Except where otherwise stated, all elements defined or mentioned
  in this specification are in the
  <code>http://www.w3.org/1999/xhtml</code> namespace, and all
  attributes defined or mentioned in this specification have no
  namespace.</p><p>Attribute names are said to be <dfn id="xml-compatible">XML-compatible</dfn> if they
  match the <a href="http://www.w3.org/TR/REC-xml/#NT-Name"><code title="">Name</code></a> production defined in XML, they contain no
  U+003A COLON characters (:), and their first three characters are
  not an <a href="#ascii-case-insensitive">ASCII case-insensitive</a> match for the string
  "<code title="">xml</code>". <a href="references.html#refsXML">[XML]</a></p><p>The term <dfn id="xml-mime-type">XML MIME type</dfn> is used to refer to the <a href="#mime-type" title="MIME type">MIME types</a> <code title="">text/xml</code>,
  <code title="">application/xml</code>, and any <a href="#mime-type">MIME
  type</a> whose subtype ends with the four characters "<code title="">+xml</code>". <a href="references.html#refsRFC3023">[RFC3023]</a></p><h4 id="dom-trees"><span class="secno">2.1.3 </span>DOM trees</h4><p>The <dfn id="root-element-of-a-document-object">root element of a <code>Document</code> object</dfn> is
  that <code><a href="#document">Document</a></code>'s first element child, if any. If it does
  not have one then the <code><a href="#document">Document</a></code> has no root element.</p><p>The term <dfn id="root-element">root element</dfn>, when not referring to a
  <code><a href="#document">Document</a></code> object's root element, means the furthest
  ancestor element node of whatever node is being discussed, or the
  node itself if it has no ancestors. When the node is a part of the
  document, then the node's <a href="#root-element">root element</a> is indeed the
  document's root element; however, if the node is not currently part
  of the document tree, the root element will be an orphaned node.</p><p>When an element's <a href="#root-element">root element</a> is the <a href="#root-element-of-a-document-object">root
  element of a <code>Document</code> object</a>, it is said to be
  <dfn id="in-a-document">in a <code>Document</code></dfn>. An element is said to have
  been <dfn id="insert-an-element-into-a-document" title="insert an element into a document">inserted into a
  document</dfn> when its <a href="#root-element">root element</a> changes and is now
  the document's <a href="#root-element">root element</a>. Analogously, an element is
  said to have been <dfn id="remove-an-element-from-a-document" title="remove an element from a
  document">removed from a document</dfn> when its <a href="#root-element">root
  element</a> changes from being the document's <a href="#root-element">root
  element</a> to being another element.</p><p>A node's <dfn id="home-subtree">home subtree</dfn> is the subtree rooted at that
  node's <a href="#root-element">root element</a>. When a node is <a href="#in-a-document">in a
  <code>Document</code></a>, its <a href="#home-subtree">home subtree</a> is that
  <code><a href="#document">Document</a></code>'s tree.</p><p>The <code><a href="#document">Document</a></code> of a <code><a href="#node">Node</a></code> (such as an
  element) is the <code><a href="#document">Document</a></code> that the <code><a href="#node">Node</a></code>'s
  <code title="dom-Node-ownerDocument"><a href="#dom-node-ownerdocument">ownerDocument</a></code> IDL
  attribute returns. When a <code><a href="#node">Node</a></code> is <a href="#in-a-document">in a
  <code>Document</code></a> then that <code><a href="#document">Document</a></code> is
  always the <code><a href="#node">Node</a></code>'s <code><a href="#document">Document</a></code>, and the
  <code><a href="#node">Node</a></code>'s <code title="dom-Node-ownerDocument"><a href="#dom-node-ownerdocument">ownerDocument</a></code> IDL attribute
  thus always returns that <code><a href="#document">Document</a></code>.</p><p>The term <dfn id="tree-order">tree order</dfn> means a pre-order, depth-first
  traversal of DOM nodes involved (through the <code title="dom-Node-parentNode"><a href="#dom-node-parentnode">parentNode</a></code>/<code title="dom-Node-childNodes"><a href="#dom-node-childnodes">childNodes</a></code> relationship).</p><p>When it is stated that some element or attribute is <dfn id="ignore" title="ignore">ignored</dfn>, or treated as some other value, or
  handled as if it was something else, this refers only to the
  processing of the node after it is in the DOM. <span class="impl">A
  user agent must not mutate the DOM in such situations.</span></p><p>The term <dfn id="text-node">text node</dfn> refers to any <code><a href="#text">Text</a></code>
  node, including <code><a href="#cdatasection">CDATASection</a></code> nodes; specifically, any
  <code><a href="#node">Node</a></code> with node type <code title="">TEXT_NODE</code> (3)
  or <code title="">CDATA_SECTION_NODE</code> (4). <a href="references.html#refsDOMCORE">[DOMCORE]</a></p><p>A content attribute is said to <dfn title="">change</dfn> value
  only if its new value is different than its previous value; setting
  an attribute to a value it already has does not change it.</p><p>The term <dfn title="">empty</dfn>, when used of an attribute
  value, text node, or string, means that the length of the text is
  zero (i.e. not even containing spaces or control characters).</p><p>Nodes can be <dfn id="concept-clone" title="concept-clone">cloned</dfn>, as
  described in the DOM Core specification. For example, the <code title="dom-cloneNode">cloneNode()</code> and <code title="dom-importNode">importNode()</code> methods of the
  <code><a href="#node">Node</a></code> interface both clone nodes, as do a number of
  algorithms in this specification. Certain <a href="#html-elements">HTML elements</a>
  (in particular, <code><a href="the-input-element.html#the-input-element">input</a></code> and <code><a href="scripting-1.html#the-script-element">script</a></code>) apply
  additional requirements on how they are cloned. <a href="references.html#refsDOMCORE">[DOMCORE]</a></p><h4 id="scripting-0"><span class="secno">2.1.4 </span>Scripting</h4><p>The construction "a <code>Foo</code> object", where
  <code>Foo</code> is actually an interface, is sometimes used instead
  of the more accurate "an object implementing the interface
  <code>Foo</code>".</p><p>An IDL attribute is said to be <dfn title="">getting</dfn> when
  its value is being retrieved (e.g. by author script), and is said to
  be <dfn title="">setting</dfn> when a new value is assigned to
  it.</p><p>If a DOM object is said to be <dfn id="live">live</dfn>, then the
  attributes and methods on that object <span class="impl">must</span>
  operate on the actual underlying data, not a snapshot of the
  data.</p><p>The terms <dfn title="">fire</dfn> and <dfn title="">dispatch</dfn> are used interchangeably in the context of
  events, as in the DOM Events specifications. The term <dfn id="concept-events-trusted" title="concept-events-trusted">trusted event</dfn> is used as
  defined by the DOM Events specification.  <a href="references.html#refsDOMEVENTS">[DOMEVENTS]</a></p><h4 id="plugins"><span class="secno">2.1.5 </span>Plugins</h4><p>The term <dfn id="plugin">plugin</dfn> refers to a user-agent defined set of
  content handlers used by the user agent that can take part in the
  user agent's rendering of a <code><a href="#document">Document</a></code> object, but that
  neither act as <a href="browsers.html#child-browsing-context" title="child browsing context">child browsing
  contexts</a> of the <code><a href="#document">Document</a></code> nor introduce any
  <code><a href="#node">Node</a></code> objects to the <code><a href="#document">Document</a></code>'s DOM.</p><p>Typically such content handlers are provided by third parties,
  though a user agent can also designate built-in content handlers as
  plugins.</p><div class="impl">

  <p>A user agent must not consider the types <code>text/plain</code>
  and <code>application/octet-stream</code> as having a registered
  <a href="#plugin">plugin</a>.</p> 
  </div><p class="example">One example of a plugin would be a PDF viewer
  that is instantiated in a <a href="browsers.html#browsing-context">browsing context</a> when the
  user navigates to a PDF file. This would count as a plugin
  regardless of whether the party that implemented the PDF viewer
  component was the same as that which implemented the user agent
  itself. However, a PDF viewer application that launches separate
  from the user agent (as opposed to using the same interface) is not
  a plugin by this definition.</p><p class="note">This specification does not define a mechanism for
  interacting with plugins, as it is expected to be user-agent- and
  platform-specific. Some UAs might opt to support a plugin mechanism
  such as the Netscape Plugin API; others might use remote content
  converters or have built-in support for certain types. Indeed, this
  specification doesn't require user agents to support plugins at all.
  <a href="references.html#refsNPAPI">[NPAPI]</a></p><div class="impl">

  <p class="warning">Browsers should take extreme care when
  interacting with external content intended for <a href="#plugin" title="plugin">plugins</a>. When third-party software is run with
  the same privileges as the user agent itself, vulnerabilities in the
  third-party software become as dangerous as those in the user
  agent.</p>

  </div><h4 id="character-encodings"><span class="secno">2.1.6 </span>Character encodings</h4><p>The <dfn id="preferred-mime-name">preferred MIME name</dfn> of a character encoding is the
  name or alias labeled as "preferred MIME name" in the IANA
  <cite>Character Sets</cite> registry, if there is one, or the
  encoding's name, if none of the aliases are so labeled. <a href="references.html#refsIANACHARSET">[IANACHARSET]</a></p><p>An <dfn id="ascii-compatible-character-encoding">ASCII-compatible character encoding</dfn> is a
  single-byte or variable-length encoding in which the bytes 0x09,
  0x0A, 0x0C, 0x0D, 0x20 - 0x22, 0x26, 0x27, 0x2C - 0x3F, 0x41 - 0x5A,
  and 0x61 - 0x7A, ignoring bytes that
  are the second and later bytes of multibyte sequences, all
  correspond to single-byte sequences that map to the same Unicode
  characters as those bytes in ANSI_X3.4-1968 (US-ASCII). <a href="references.html#refsRFC1345">[RFC1345]</a></p><p class="note">This includes such encodings as Shift_JIS,
  HZ-GB-2312, and variants of ISO-2022, even though it is possible in
  these encodings for bytes like 0x70 to be part of longer sequences
  that are unrelated to their interpretation as ASCII. It excludes
  such encodings as UTF-7, UTF-16, GSM03.38, and EBCDIC variants.</p><p>The term <dfn title="">Unicode character</dfn> is used to mean a
  <i title="">Unicode scalar value</i> (i.e. any Unicode code point
  that is not a surrogate code point). <a href="references.html#refsUNICODE">[UNICODE]</a></p><h3 id="conformance-requirements"><span class="secno">2.2 </span>Conformance requirements</h3><p>All diagrams, examples, and notes in this specification are
  non-normative, as are all sections explicitly marked non-normative.
  Everything else in this specification is normative.</p><p>The key words "MUST", "MUST NOT", "REQUIRED",  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
  "OPTIONAL" in the normative parts of this document are to be
  interpreted as described in RFC2119. For readability, these words do
  not appear in all uppercase letters in this specification. <a href="references.html#refsRFC2119">[RFC2119]</a></p><div class="impl">

  <p>Requirements phrased in the imperative as part of algorithms
  (such as "strip any leading space characters" or "return false and
  abort these steps") are to be interpreted with the meaning of the
  key word ("must", "should", "may", etc) used in introducing the
  algorithm.</p>

  <p>Conformance requirements phrased as algorithms or specific steps
  may be implemented in any manner, so long as the end result is
  equivalent. (In particular, the algorithms defined in this
  specification are intended to be easy to follow, and not intended to
  be performant.)</p>

  </div><div class="impl">

  <h4 id="conformance-classes"><span class="secno">2.2.1 </span>Conformance classes</h4>

  <p>This specification describes the conformance criteria for <span class="impl">user agents (relevant to implementors) and</span>
  documents<span class="impl"> (relevant to authors and authoring tool
  implementors)</span>.</p>

  <p><dfn id="conforming-html5-documents">Conforming
  HTML5
  documents</dfn> are those that comply with all
  the conformance criteria for documents. For readability, some of
  these conformance requirements are phrased as conformance
  requirements on authors; such requirements are implicitly
  requirements on documents: by definition, all documents are assumed
  to have had an author. (In some cases, that author may itself be a
  user agent &#8212; such user agents are subject to additional rules,
  as explained below.)</p>

  <p class="example">For example, if a requirement states that
  "authors must not use the <code title="">foobar</code> element", it
  would imply that documents are not allowed to contain elements named
  <code title="">foobar</code>.</p>

  <p class="note">the conformance requirements for documents include
  syntax (the &lt;table&gt; element is conforming as a child of
  &lt;body&gt;, but not as a child ot &lt;title&gt;), and semantics (the
  &lt;table&gt; elements denotes a multi-dimensional data table, not a
  piece of furniture).</p>

  <p class="note impl">There is no implied relationship between
  document conformance requirements and implementation conformance
  requirements. User agents are not free to handle non-conformant
  documents as they please; the processing model described in this
  specification applies to implementations regardless of the
  conformity of the input documents.</p>

  <p>User agents fall into several (overlapping) categories with
  different conformance requirements.</p>

  <dl><dt id="interactive">Web browsers and other interactive user agents</dt>

   <dd>

    <p>Web browsers that support <a href="the-xhtml-syntax.html#the-xhtml-syntax">the XHTML syntax</a> must
    process elements and attributes from the <a href="namespaces.html#html-namespace-0">HTML
    namespace</a> found in XML documents as described in this
    specification, so that users can interact with them, unless the
    semantics of those elements have been overridden by other
    specifications.</p>

    <p class="example">A conforming XHTML processor would, upon
    finding an XHTML <code><a href="scripting-1.html#the-script-element">script</a></code> element in an XML document,
    execute the script contained in that element. However, if the
    element is found within a transformation expressed in XSLT
    (assuming the user agent also supports XSLT), then the processor
    would instead treat the <code><a href="scripting-1.html#the-script-element">script</a></code> element as an opaque
    element that forms part of the transform.</p>

    <p>Web browsers that support <a href="syntax.html#syntax">the HTML syntax</a> must
    process documents labeled with an <a href="#html-mime-type">HTML MIME type</a> as
    described in this specification, so that users can interact with
    them.</p>

    <p>User agents that support scripting must also be conforming
    implementations of the IDL fragments in this specification, as
    described in the Web IDL specification. <a href="references.html#refsWEBIDL">[WEBIDL]</a></p>

    <p class="note">Unless explicitly stated, specifications that
    override the semantics of HTML elements do not override the
    requirements on DOM objects representing those elements. For
    example, the <code><a href="scripting-1.html#the-script-element">script</a></code> element in the example above
    would still implement the <code><a href="scripting-1.html#htmlscriptelement">HTMLScriptElement</a></code>
    interface.</p>

   </dd>

   <dt id="non-interactive">Non-interactive presentation user agents</dt>

   <dd>

    <p>User agents that process HTML and XHTML documents purely to
    render non-interactive versions of them must comply to the same
    conformance criteria as Web browsers, except that they are exempt
    from requirements regarding user interaction.</p>

    <p class="note">Typical examples of non-interactive presentation
    user agents are printers (static UAs) and overhead displays
    (dynamic UAs). It is expected that most static non-interactive
    presentation user agents will also opt to <a href="#non-scripted">lack scripting support</a>.</p>

    <p class="example">A non-interactive but dynamic presentation UA
    would still execute scripts, allowing forms to be dynamically
    submitted, and so forth. However, since the concept of "focus" is
    irrelevant when the user cannot interact with the document, the UA
    would not need to support any of the focus-related DOM APIs.</p>

   </dd>

   <dt id="renderingUA">Visual user agents that support the suggested default rendering</dt>

   <dd>

    <p>User agents, whether interactive or not, may be designated
    (possibly as a user option) as supporting the suggested default
    rendering defined by this specification.</p>

    <p>User agents that are designated as supporting the suggested
    default rendering must implement the rules in <a href="rendering.html#rendering">the rendering section</a> that that section
    defines as the behavior that user agents are <em>expected</em> to
    implement.</p>

   </dd>

   <dt id="non-scripted">User agents with no scripting support</dt>

   <dd>

    <p>Implementations that do not support scripting (or which have
    their scripting features disabled entirely) are exempt from
    supporting the events and DOM interfaces mentioned in this
    specification. For the parts of this specification that are
    defined in terms of an events model or in terms of the DOM, such
    user agents must still act as if events and the DOM were
    supported.</p>

    <p class="note">Scripting can form an integral part of an
    application. Web browsers that do not support scripting, or that
    have scripting disabled, might be unable to fully convey the
    author's intent.</p>

   </dd>


   <dt>Conformance checkers</dt>

   <dd id="conformance-checkers">

    <p>Conformance checkers must verify that a document conforms to
    the applicable conformance criteria described in this
    specification. Automated conformance checkers are exempt from
    detecting errors that require interpretation of the author's
    intent (for example, while a document is non-conforming if the
    content of a <code><a href="grouping-content.html#the-blockquote-element">blockquote</a></code> element is not a quote,
    conformance checkers running without the input of human judgement
    do not have to check that <code><a href="grouping-content.html#the-blockquote-element">blockquote</a></code> elements only
    contain quoted material).</p>

    <p>Conformance checkers must check that the input document
    conforms when parsed without a <a href="browsers.html#browsing-context">browsing context</a>
    (meaning that no scripts are run, and that the parser's
    <a href="parsing.html#scripting-flag">scripting flag</a> is disabled), and should also check
    that the input document conforms when parsed with a <a href="browsers.html#browsing-context">browsing
    context</a> in which scripts execute, and that the scripts
    never cause non-conforming states to occur other than transiently
    during script execution itself. (This is only a "SHOULD" and not a
    "MUST" requirement because it has been proven to be impossible. <a href="references.html#refsCOMPUTABLE">[COMPUTABLE]</a>)</p>

    
    <p>The term "HTML5 validator" can be used to refer to a
    conformance checker that itself conforms to the applicable
    requirements of this specification.</p>

    <div class="note">

     <p>XML DTDs cannot express all the conformance requirements of
     this specification. Therefore, a validating XML processor and a
     DTD cannot constitute a conformance checker. Also, since neither
     of the two authoring formats defined in this specification are
     applications of SGML, a validating SGML system cannot constitute
     a conformance checker either.</p>

     <p>To put it another way, there are three types of conformance
     criteria:</p>

     <ol><li>Criteria that can be expressed in a DTD.</li>

      <li>Criteria that cannot be expressed by a DTD, but can still be
      checked by a machine.</li>

      <li>Criteria that can only be checked by a human.</li>

     </ol><p>A conformance checker must check for the first two. A simple
     DTD-based validator only checks for the first class of errors and
     is therefore not a conforming conformance checker according to
     this specification.</p>

    </div>
   </dd>


   <dt>Data mining tools</dt>

   <dd id="data-mining">

    <p>Applications and tools that process HTML and XHTML documents
    for reasons other than to either render the documents or check
    them for conformance should act in accordance with the semantics
    of the documents that they process.</p>

    <p class="example">A tool that generates <a href="sections.html#outline" title="outline">document outlines</a> but increases the nesting
    level for each paragraph and does not increase the nesting level
    for each section would not be conforming.</p>

   </dd>


   <dt id="editors">Authoring tools and markup generators</dt>

   <dd>

    <p>Authoring tools and markup generators must generate
    <a href="#conforming-html5-documents">conforming
    HTML5
    documents</a>. Conformance criteria that apply to authors also
    apply to authoring tools, where appropriate.</p>

    <p>Authoring tools are exempt from the strict requirements of
    using elements only for their specified purpose, but only to the
    extent that authoring tools are not yet able to determine author
    intent. However, authoring tools must not automatically misuse
    elements or encourage their users to do so.</p>

    <p class="example">For example, it is not conforming to use an
    <code><a href="sections.html#the-address-element">address</a></code> element for arbitrary contact information;
    that element can only be used for marking up contact information
    for the author of the document or section. However, since an
    authoring tool is likely unable to determine the difference, an
    authoring tool is exempt from that requirement. This does not
    mean, though, that authoring tools can use <code><a href="sections.html#the-address-element">address</a></code>
    elements for any block of italics text (for instance); it just
    means that the authoring tool doesn't have to verify that when the
    user uses a tool for inserting contact information for a section,
    that the user really is doing that and not inserting something
    else instead.</p>

    <p class="note">In terms of conformance checking, an editor has to
    output documents that conform to the same extent that a
    conformance checker will verify.</p>

    <p>When an authoring tool is used to edit a non-conforming
    document, it may preserve the conformance errors in sections of
    the document that were not edited during the editing session
    (i.e. an editing tool is allowed to round-trip erroneous
    content). However, an authoring tool must not claim that the
    output is conformant if errors have been so preserved.</p>

    <p>Authoring tools are expected to come in two broad varieties:
    tools that work from structure or semantic data, and tools that
    work on a What-You-See-Is-What-You-Get media-specific editing
    basis (WYSIWYG).</p>

    <p>The former is the preferred mechanism for tools that author
    HTML, since the structure in the source information can be used to
    make informed choices regarding which HTML elements and attributes
    are most appropriate.</p>

    <p>However, WYSIWYG tools are legitimate. WYSIWYG tools should use
    elements they know are appropriate, and should not use elements
    that they do not know to be appropriate. This might in certain
    extreme cases mean limiting the use of flow elements to just a few
    elements, like <code><a href="grouping-content.html#the-div-element">div</a></code>, <code><a href="text-level-semantics.html#the-b-element">b</a></code>, <code><a href="text-level-semantics.html#the-i-element">i</a></code>,
    and <code><a href="text-level-semantics.html#the-span-element">span</a></code> and making liberal use of the <code title="attr-style"><a href="elements.html#the-style-attribute">style</a></code> attribute.</p>

    <p>All authoring tools, whether WYSIWYG or not, should make a best
    effort attempt at enabling users to create well-structured,
    semantically rich, media-independent content.</p>

   </dd>

  </dl><p id="hardwareLimitations">User agents may impose
  implementation-specific limits on otherwise unconstrained inputs,
  e.g. to prevent denial of service attacks, to guard against running
  out of memory, or to work around platform-specific limitations.</p>

  <p>For compatibility with existing content and prior specifications,
  this specification describes two authoring formats: one based on XML
  (referred to as <a href="the-xhtml-syntax.html#the-xhtml-syntax">the XHTML syntax</a>), and one using a <a href="syntax.html#writing">custom format</a> inspired by SGML (referred to as
  <a href="syntax.html#syntax">the HTML syntax</a>). Implementations must support at least
  one of these two formats, although supporting both is
  encouraged.</p>

  <p id="entity-references">The language in this specification assumes
  that the user agent expands all entity references, and therefore
  does not include entity reference nodes in the DOM. If user agents
  do include entity reference nodes in the DOM, then user agents must
  handle them as if they were fully expanded when implementing this
  specification. For example, if a requirement talks about an
  element's child text nodes, then any text nodes that are children of
  an entity reference that is a child of that element would be used as
  well. Entity references to unknown entities must be treated as if
  they contained just an empty text node for the purposes of the
  algorithms defined in this specification.</p>

  <p>Some conformance requirements are phrased as requirements on
  elements, attributes, methods or objects. Such requirements fall
  into two categories: those describing content model restrictions,
  and those describing implementation behavior. Those in the former
  category are requirements on documents and authoring tools. Those in
  the second category are requirements on user agents. Similarly, some
  conformance requirements are phrased as requirements on authors;
  such requirements are to be interpreted as conformance requirements
  on the documents that authors produce. (In other words, this
  specification does not distinguish between conformance criteria on
  authors and conformance criteria on documents.)</p>

  </div><div class="impl">

  <h4 id="dependencies"><span class="secno">2.2.2 </span>Dependencies</h4>

  <p>This specification relies on several other underlying
  specifications.</p>

  <dl><dt>XML</dt>

   <dd>

    <p>Implementations that support <a href="the-xhtml-syntax.html#the-xhtml-syntax">the XHTML syntax</a> must
    support some version of XML, as well as its corresponding
    namespaces specification, because that syntax uses an XML
    serialization with namespaces. <a href="references.html#refsXML">[XML]</a> <a href="references.html#refsXMLNS">[XMLNS]</a></p>

   </dd>

   <dt>DOM</dt>

   <dd>

    <p>The Document Object Model (DOM) is a representation &#8212; a
    model &#8212; of a document and its content. The DOM is not just
    an API; the conformance criteria of HTML implementations are
    defined, in this specification, in terms of operations on the DOM.
    <a href="references.html#refsDOMCORE">[DOMCORE]</a></p>

    <p>Implementations must support some version of DOM Core and DOM
    Events, because this specification is defined in terms of the DOM,
    and some of the features are defined as extensions to the DOM Core
    interfaces. <a href="references.html#refsDOMCORE">[DOMCORE]</a> <a href="references.html#refsDOMEVENTS">[DOMEVENTS]</a></p>

    <p>In particular, the following features are defined in the DOM
    Core specification: <a href="references.html#refsDOMCORE">[DOMCORE]</a></p>

    <ul class="brief"><li><dfn id="attr"><code>Attr</code></dfn> interface</li>
     <li><dfn id="cdatasection"><code>CDATASection</code></dfn> interface</li>
     <li><dfn id="comment-0"><code>Comment</code></dfn> interface</li>
     <li><dfn id="domimplementation"><code>DOMImplementation</code></dfn> interface</li>
     <li><dfn id="document"><code>Document</code></dfn> interface</li>
     <li><dfn id="documentfragment"><code>DocumentFragment</code></dfn> interface</li>
     <li><dfn id="documenttype"><code>DocumentType</code></dfn> interface</li>
     <li><dfn id="domexception"><code>DOMException</code></dfn> interface</li>
     <li><dfn id="element"><code>Element</code></dfn> interface</li>
     <li><dfn id="node"><code>Node</code></dfn> interface</li>
     <li><dfn id="nodelist"><code>NodeList</code></dfn> interface</li>
     <li><dfn id="processinginstruction"><code>ProcessingInstruction</code></dfn> interface</li>
     <li><dfn id="text"><code>Text</code></dfn> interface</li>

     <li><dfn id="dom-domimplementation-createdocument" title="dom-DOMImplementation-createDocument"><code>createDocument()</code></dfn> method</li>
     <li><dfn id="dom-document-createelement" title="dom-Document-createElement"><code>createElement()</code></dfn> method</li>
     <li><dfn id="dom-document-createelementns" title="dom-Document-createElementNS"><code>createElementNS()</code></dfn> method</li>
     <li><dfn id="dom-document-getelementbyid" title="dom-Document-getElementById"><code>getElementById()</code></dfn> method</li>
     <li><dfn id="dom-node-insertbefore" title="dom-Node-insertBefore"><code>insertBefore()</code></dfn> method</li>

     <li><dfn id="dom-node-ownerdocument" title="dom-Node-ownerDocument"><code>ownerDocument</code></dfn> attribute</li>
     <li><dfn id="dom-node-childnodes" title="dom-Node-childNodes"><code>childNodes</code></dfn> attribute</li>
     <li><dfn id="dom-node-localname" title="dom-Node-localName"><code>localName</code></dfn> attribute</li>
     <li><dfn id="dom-node-parentnode" title="dom-Node-parentNode"><code>parentNode</code></dfn> attribute</li>
     <li><dfn id="dom-node-namespaceuri" title="dom-Node-namespaceURI"><code>namespaceURI</code></dfn> attribute</li>
     <li><dfn id="dom-element-tagname" title="dom-Element-tagName"><code>tagName</code></dfn> attribute</li>
     <li><dfn id="textcontent"><code>textContent</code></dfn> attribute</li>

    </ul><p>The following features are defined in the DOM Events
    specification: <a href="references.html#refsDOMEVENTS">[DOMEVENTS]</a></p>

    <ul class="brief"><li><dfn id="event"><code>Event</code></dfn> interface</li>
     <li><dfn id="eventtarget"><code>EventTarget</code></dfn> interface</li>
     <li><dfn id="uievent"><code>UIEvent</code></dfn> interface</li>
     <li><dfn id="mouseevent"><code>MouseEvent</code></dfn> interface</li>

     <li><dfn id="event-click" title="event-click"><code>click</code></dfn> event</li>

     <li><dfn id="dom-event-target" title="dom-Event-target"><code>target</code></dfn> attribute</li>

    </ul></dd>

   <dt>File API</dt>

   <dd>

    <p>This specification uses the following interfaces defined in the
    File API specification: <a href="references.html#refsFILEAPI">[FILEAPI]</a></p>

    <ul class="brief"><li><dfn id="blob">Blob</dfn></li>
     <li><dfn id="file">File</dfn></li>
     <li><dfn id="filelist">FileList</dfn></li>

    </ul></dd>

   <dt>Web IDL</dt>

   <dd>

    <p>The IDL fragments in this specification must be interpreted as
    required for conforming IDL fragments, as described in the Web IDL
    specification. <a href="references.html#refsWEBIDL">[WEBIDL]</a></p>

    <p>The terms <dfn id="supported-property-indices">supported property indices</dfn> and
    <dfn id="supported-property-names">supported property names</dfn> are used as defined in the
    WebIDL specification.</p>

    <p id="float-nan">Except where otherwise specified, if an IDL
    attribute that is a floating point number type (<code title="">double</code>) is assigned an Infinity or Not-a-Number
    (NaN) value, a <code><a href="common-dom-interfaces.html#not_supported_err">NOT_SUPPORTED_ERR</a></code> exception must be
    raised.</p>

    <p>Except where otherwise specified, if a method with an argument
    that is a floating point number type (<code title="">double</code>)
    is passed an Infinity or Not-a-Number (NaN) value, a
    <code><a href="common-dom-interfaces.html#not_supported_err">NOT_SUPPORTED_ERR</a></code> exception must be raised.</p>

    <p>Except where otherwise specified, if a method has an argument
    of type <code>DOMString</code>, or if an IDL attribute is assigned
    a new value of type <code>DOMString</code>, the user agent must
    <span title="dfn-obtain-unicode">convert the
    <code>DOMString</code> to a sequence of Unicode characters</span>
    to obtain the string on which the algorithms in this specification
    are to operate. <a href="references.html#refsWEBIDL">[WEBIDL]</a></p>

   </dd>

   <dt>JavaScript</dt>

   <dd>

    <p>Some parts of the language described by this specification only
    support JavaScript as the underlying scripting language. <a href="references.html#refsECMA262">[ECMA262]</a></p>

    <p class="note">The term "JavaScript" is used to refer to ECMA262,
    rather than the official term ECMAScript, since the term
    JavaScript is more widely known. Similarly, the <a href="#mime-type">MIME
    type</a> used to refer to JavaScript in this specification is
    <code title="">text/javascript</code>, since that is the most
    commonly used type, <a href="introduction.html#willful-violation" title="willful violation">despite it
    being an officially obsoleted type</a> according to RFC
    4329. <a href="references.html#refsRFC4329">[RFC4329]</a></p>

   </dd>

   <dt>Media Queries</dt>

   <dd>

    <p>Implementations must support some version of the Media Queries
    language. <a href="references.html#refsMQ">[MQ]</a></p>

   </dd>

   <dt>URIs, IRIs, IDNA</dt>

   <dd>

    <p>Implementations must support the semantics of <a href="urls.html#url" title="URL">URLs</a> defined in the URI and IRI specifications,
    as well as the semantics of IDNA domain names defined in the
    <cite>Internationalizing Domain Names in Applications
    (IDNA)</cite> specification. <a href="references.html#refsRFC3986">[RFC3986]</a>
    <a href="references.html#refsRFC3987">[RFC3987]</a> <a href="references.html#refsRFC3490">[RFC3490]</a>

   </p></dd>

   <dt>CSS modules</dt>

   <dd>

    <p>While support for CSS as a whole is not required of
    implementations of this specification (though it is encouraged, at
    least for Web browsers), some features are defined in terms of
    specific CSS requirements.</p>

    <p>In particular, some features require that a string be
    <dfn id="parsed-as-a-css-color-value">parsed as a CSS &lt;color&gt; value</dfn>. When parsing a CSS
    value, user agents are required by the CSS specifications to apply
    some error handling rules. These apply to this specification also.
    <a href="references.html#refsCSSCOLOR">[CSSCOLOR]</a> <a href="references.html#refsCSS">[CSS]</a></p>

    <p class="example">For example, user agents are required to close
    all open constructs upon finding the end of a style sheet
    unexpectedly. Thus, when parsing the string "<code title="">rgb(0,0,0</code>" (with a missing close-parenthesis) for
    a color value, the close parenthesis is implied by this error
    handling rule, and a value is obtained (the color 'black').
    However, the similar construct "<code title="">rgb(0,0,</code>"
    (with both a missing parenthesis and a missing "blue" value)
    cannot be parsed, as closing the open construct does not result
    in a viable value.</p>

   </dd>

  </dl><p>This specification does not <em>require</em> support of any
  particular network protocol, style sheet language, scripting
  language, or any of the DOM specifications beyond those described
  above. However, the language described by this specification is
  biased towards CSS as the styling language, JavaScript as the
  scripting language, and HTTP as the network protocol, and several
  features assume that those languages and protocols are in use.</p>

  <p class="note">This specification might have certain additional
  requirements on character encodings, image formats, audio formats,
  and video formats in the respective sections.</p>

  </div><h4 id="extensibility"><span class="secno">2.2.3 </span>Extensibility</h4><p>HTML has a wide number of extensibility mechanisms that can be
  used for adding semantics in a safe manner:</p><ul><li>Authors can use the <code title="attr-class"><a href="elements.html#classes">class</a></code>
   attribute to extend elements, effectively creating their own
   elements, while using the most applicable existing "real" HTML
   element, so that browsers and other tools that don't know of the
   extension can still support it somewhat well. This is the tack used
   by Microformats, for example.</li>

   <li>Authors can include data for inline client-side scripts or
   server-side site-wide scripts to process using the <code title="attr-data-*"><a href="elements.html#attr-data">data-*=""</a></code> attributes. These are
   guaranteed to never be touched by browsers, and allow scripts to
   include data on HTML elements that scripts can then look for and
   process.</li>

   <li>Authors can use the <code title="meta"><a href="semantics.html#the-meta-element">&lt;meta name=""
   content=""&gt;</a></code> mechanism to include page-wide metadata by
   registering <a href="semantics.html#concept-meta-extensions" title="concept-meta-extensions">extensions to the
   predefined set of metadata names</a>.</li>

   <li>Authors can use the <code title="attr-hyperlink-rel"><a href="links.html#attr-hyperlink-rel">rel=""</a></code> mechanism to annotate
   links with specific meanings by registering <a href="links.html#concept-rel-extensions" title="concept-rel-extensions">extensions to the predefined set of
   link types</a>. This is also used by Microformats.</li>

   <li>Authors can embed raw data using the <code title="script"><a href="scripting-1.html#the-script-element">&lt;script type=""&gt;</a></code> mechanism with a custom
   type, for further handling by inline or server-side scripts.</li>

   <li>Authors can create <a href="#plugin" title="plugin">plugins</a> and
   invoke them using the <code><a href="the-iframe-element.html#the-embed-element">embed</a></code> element. This is how Flash
   works.</li>

   <li>Authors can extend APIs using the JavaScript prototyping
   mechanism. This is widely used by script libraries, for
   instance.</li>


  </ul><div class="impl">

  <hr><p>Vendor-specific proprietary user agent extensions to this
  specification are strongly discouraged. Documents must not use such
  extensions, as doing so reduces interoperability and fragments the
  user base, allowing only users of specific user agents to access the
  content in question.</p>

  <p>If such extensions are nonetheless needed, e.g. for experimental
  purposes, then vendors are strongly urged to use one of the
  following extension mechanisms:</p>

  <p>For markup-level features that can be limited to the XML
  serialization and need not be supported in the HTML serialization,
  vendors should use the namespace mechanism to define custom
  namespaces in which the non-standard elements and attributes are
  supported.</p>

  <p>For markup-level features that are intended for use with
  <a href="syntax.html#syntax">the HTML syntax</a>, extensions should be limited to new
  attributes of the form "<code title="">x-<var title="">vendor</var>-<var title="">feature</var></code>", where
  <var title="">vendor</var> is a short string that identifies the
  vendor responsible for the extension, and <var title="">feature</var> is the name of the feature. New element names
  should not be created. Using attributes for such extensions
  exclusively allows extensions from multiple vendors to co-exist on
  the same element, which would not be possible with elements. Using
  the "<code title="">x-<var title="">vendor</var>-<var title="">feature</var></code>" form allows extensions to be made
  without risk of conflicting with future additions to the
  specification.</p>

  <div class="example">

   <p>For instance, a browser named "FerretBrowser" could use "ferret"
   as a vendor prefix, while a browser named "Mellblom Browser" could
   use "mb". If both of these browsers invented extensions that turned
   elements into scratch-and-sniff areas, an author experimenting with
   these features could write:</p>

   <pre>&lt;p&gt;This smells of lemons!
&lt;span x-ferret-smellovision x-ferret-smellcode="LEM01"
      x-mb-outputsmell x-mb-smell="lemon juice"&gt;&lt;/span&gt;&lt;/p&gt;</pre>

  </div>

  <p>Attribute names beginning with the two characters "<code title="">x-</code>" are reserved for user agent use and are
  guaranteed to never be formally added to the HTML language. For
  flexibility, attributes names containing underscores (the U+005F LOW
  LINE character) are also reserved for experimental purposes and are
  guaranteed to never be formally added to the HTML language.</p>

  <p class="note">Pages that use such attributes are by definition
  non-conforming.</p>

  <p>For DOM extensions, e.g. new methods and IDL attributes, the new
  members should be prefixed by vendor-specific strings to prevent
  clashes with future versions of this specification.</p>

  <p>For events, experimental event names should be prefixed with
  vendor-specific strings.</p>

  <div class="example">

   <p>For example, if a user agent called "Pleasold" were to
   add an event to indicate when the user is going up in an elevator,
   it could use the prefix "<code title="">pleasold</code>" and thus
   name the event "<code title="">pleasoldgoingup</code>", possibly
   with an event handler attribute named "<code title="">onpleasoldgoingup</code>".</p>

  </div>

  <p>All extensions must be defined so that the use of extensions
  neither contradicts nor causes the non-conformance of functionality
  defined in the specification.</p> 

  <div class="example">

   <p>For example, while strongly discouraged from doing so, an
   implementation "Foo Browser" could add a new IDL attribute "<code title="">fooTypeTime</code>" to a control's DOM interface that
   returned the time it took the user to select the current value of a
   control (say). On the other hand, defining a new control that
   appears in a form's <code title="dom-form-elements"><a href="forms.html#dom-form-elements">elements</a></code>
   array would be in violation of the above requirement, as it would
   violate the definition of <code title="dom-form-elements"><a href="forms.html#dom-form-elements">elements</a></code> given in this
   specification.</p>

  </div>

  <p>When adding new <a href="common-dom-interfaces.html#reflect" title="reflect">reflecting</a> IDL
  attributes corresponding to content attributes of the form "<code title="">x-<var title="">vendor</var>-<var title="">feature</var></code>", the IDL attribute should be named
  "<code title=""><var title="">vendor</var><var title="">Feature</var></code>" (i.e. the "<code title="">x</code>"
  is dropped from the IDL attribute's name).</p>

  <hr><p>When vendor-neutral extensions to this specification are needed,
  either this specification can be updated accordingly, or an
  extension specification can be written that overrides the
  requirements in this specification. When someone applying this
  specification to their activities decides that they will recognize
  the requirements of such an extension specification, it becomes an
  <dfn id="other-applicable-specifications" title="other applicable specifications">applicable
  specification</dfn>.

  </p><p>The conformance terminology for documents depends on the nature
  of the changes introduced by such applicable specificactions, and on
  the content and intended interpretation of the document. Applicable
  specifications MAY define new document content (e.g. a foobar
  element), MAY prohibit certain otherwise conforming content (e.g.
  prohibit use of &lt;table&gt;s), or MAY change the semantics, DOM
  mappings, or other processing rules for content defined in this
  specification. Whether a document is or is not a <a href="#conforming-html5-documents">conforming HTML5 document</a> does not
  depend on the use of applicable specifications: if the syntax and
  semantics of a given <a href="#conforming-html5-documents">conforming
  HTML5 document </a>document is unchanged by the use of applicable
  specification(s), then that document remains a <a href="#conforming-html5-documents">conforming HTML5 document</a>. If the
  semantics or processing of a given (otherwise conforming) document
  is changed by use of applicable specification(s), then it is not a
  <a href="#conforming-html5-documents">conforming HTML5 document</a>. For
  such cases, the applicable specifications SHOULD define conformance
  terminology.</p>

  <p class="note">As a suggested but not required convention, such
  specifications might define conformance terminology such as:
  "Conforming HTML5+XXX document", where XXX is a short
  name for the applicable specification. (Example: "Conforming
  HTML5+AutomotiveExtensions document").</p>
 
  <p class="note">a consequence of the rule given above is that
  certain syntactically correct HTML5 documents may not be <a href="#conforming-html5-documents">conforming HTML5 documents</a> in the
  presence of applicable specifications. (Example: the applicable
  specification defines &lt;table&gt; to be a piece of furniture &#8212;
  a document written to that specification and containing a &lt;table&gt;
  element is NOT a <a href="#conforming-html5-documents">conforming HTML5
  document</a>, even if the element happens to be syntactically
  correct HTML5.)</p>

  <hr><p>User agents must treat elements and attributes that they do not
  understand as semantically neutral; leaving them in the DOM (for DOM
  processors), and styling them according to CSS (for CSS processors),
  but not inferring any meaning from them.</p>

  <p>When support for a feature is disabled (e.g. as an emergency
  measure to mitigate a security problem, or to aid in development, or
  for performance reasons), user agents must act as if they had no
  support for the feature whatsoever, and as if the feature was not
  mentioned in this specification. For example, if a particular
  feature is accessed via an attribute in a Web IDL interface, the
  attribute itself would be omitted from the objects that implement
  that interface &#8212; leaving the attribute on the object but
  making it return null or throw an exception is insufficient.</p>

  </div><h3 id="case-sensitivity-and-string-comparison"><span class="secno">2.3 </span>Case-sensitivity and string comparison</h3><p>Comparing two strings in a <dfn id="case-sensitive">case-sensitive</dfn> manner means
  comparing them exactly, code point for code point.</p><p>Comparing two strings in an <dfn id="ascii-case-insensitive">ASCII case-insensitive</dfn>
  manner means comparing them exactly, code point for code point, except
  that the characters in the range U+0041 to U+005A (i.e. LATIN
  CAPITAL LETTER A to LATIN CAPITAL LETTER Z) and the corresponding
  characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A
  to LATIN SMALL LETTER Z) are considered to also match.</p><p>Comparing two strings in a <dfn id="compatibility-caseless">compatibility caseless</dfn>
  manner means using the Unicode <i>compatibility caseless match</i>
  operation to compare the two strings. <a href="references.html#refsUNICODE">[UNICODE]</a></p><p>Except where otherwise stated, string comparisons must be
  performed in a <a href="#case-sensitive">case-sensitive</a> manner.</p><div class="impl">

  <p><dfn id="converted-to-ascii-uppercase" title="converted to ASCII uppercase">Converting a string to
  ASCII uppercase</dfn> means replacing all characters in the range
  U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z)
  with the corresponding characters in the range U+0041 to U+005A
  (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z).</p>

  <p><dfn id="converted-to-ascii-lowercase" title="converted to ASCII lowercase">Converting a string to
  ASCII lowercase</dfn> means replacing all characters in the range
  U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL
  LETTER Z) with the corresponding characters in the range U+0061
  to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z).</p>

  </div><p>A string <var title="">pattern</var> is a <dfn id="prefix-match">prefix match</dfn>
  for a string <var title="">s</var> when <var title="">pattern</var>
  is not longer than <var title="">s</var> and truncating <var title="">s</var> to <var title="">pattern</var>'s length leaves the
  two strings as matches of each other.</p><div class="impl">

  <h3 id="utf-8"><span class="secno">2.4 </span>UTF-8</h3>

  <p>When a user agent is required to <dfn id="decoded-as-utf-8-with-error-handling" title="decoded as UTF-8,
  with error handling">decode a byte string as UTF-8, with error
  handling</dfn>, it means that the byte stream must be converted to a
  Unicode string by interpreting it as UTF-8, except that any errors
  must be handled as described in the following list. Bytes in the
  following list are represented in hexadecimal. <a href="references.html#refsRFC3629">[RFC3629]</a>

  </p><dl class="switch"><dt>One byte in the range FE to FF</dt>


   <dt><a href="#overlong-form" title="overlong form">Overlong forms</a> (e.g. F0 80 80 A0)</dt>

   <dt>One byte in the range C0 to C1, followed by one byte in the range 80 to BF</dt> 


   <dt>One byte in the range F0 to F4, followed by three bytes in the range 80 to BF that represent a code point above U+10FFFF</dt>

   <dt>One byte in the range F5 to F7, followed by three bytes in the range 80 to BF</dt> 

   <dt>One byte in the range F8 to FB, followed by four bytes in the range 80 to BF</dt> 

   <dt>One byte in the range FC to FD, followed by five bytes in the range 80 to BF</dt> 


   <dt>One byte in the range C0 to FD that is not followed by a byte in the range 80 to BF</dt> 

   <dt>One byte in the range E0 to FD, followed by a byte in the range 80 to BF that is not followed by a byte in the range 80 to BF</dt> 

   <dt>One byte in the range F0 to FD, followed by two bytes in the range 80 to BF, the last of which is not followed by a byte in the range 80 to BF</dt> 

   <dt>One byte in the range F8 to FD, followed by three bytes in the range 80 to BF, the last of which is not followed by a byte in the range 80 to BF</dt> 

   <dt>One byte in the range FC to FD, followed by four bytes in the range 80 to BF, the last of which is not followed by a byte in the range 80 to BF</dt> 


   <dt>Any byte sequence that represents a code point in the range U+D800 to U+DFFF</dt> 


   <dd>The whole matched sequence must be replaced by a single U+FFFD
   REPLACEMENT CHARACTER.</dd>


   <dt>One byte in the range 80 to BF not preceded by a byte in the range 80 to FD</dt>

   <dt>One byte in the range 80 to BF preceded by a byte that is part of a complete UTF-8 sequence that does not include this byte</dt>

   <dt>One byte in the range 80 to BF preceded by a byte that is part of a sequence that has been replaced by a U+FFFD REPLACEMENT CHARACTER, either alone or as part of a sequence</dt>

   <dd>Each such byte must be replaced with a U+FFFD REPLACEMENT CHARACTER.</dd>


  </dl><p>For the purposes of the above requirements, an <dfn id="overlong-form">overlong
  form</dfn> in UTF-8 is a sequence that encodes a code point using
  more bytes than the minimum needed to encode that code point in
  UTF-8.</p>

  <p class="example">For example, the byte string "41 98 BA 42 E2 98
  43 E2 98 BA E2 98" would be converted to the string
  "A&#65533;&#65533;B&#65533;C&#9786;&#65533;".</p>

  </div></body></html>