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<p><a href="http://www.w3.org/"><img alt="W3C" height="48" src="http://www.w3.org/Icons/w3c_home" width="72"></a></p>
<h1>Server-Sent Events</h1>
<h2 class="no-num no-toc" id="editor-s-draft-20-october-2011">W3C Working Draft 20 October 2011</h2>
<dl>
<dt>This Version:</dt>
<dd><a href="http://www.w3.org/TR/2011/WD-eventsource-20111020/">http://www.w3.org/TR/2011/WD-eventsource-20111020/</a></dd>
<dt>Latest Published Version:</dt>
<dd><a href="http://www.w3.org/TR/eventsource/">http://www.w3.org/TR/eventsource/</a></dd>
<dt>Latest Editor's Draft:</dt>
<dd><a class="latest-link" href="http://dev.w3.org/html5/eventsource/">http://dev.w3.org/html5/eventsource/</a></dd>
<dt>Previous Versions:</dt>
<dd><a href="http://www.w3.org/TR/2011/WD-eventsource-20110310/">http://www.w3.org/TR/2011/WD-eventsource-20110310/</a></dd>
<dd><a href="http://www.w3.org/TR/2011/WD-eventsource-20110208/">http://www.w3.org/TR/2011/WD-eventsource-20110208/</a></dd>
<dd><a href="http://www.w3.org/TR/2009/WD-eventsource-20091222/">http://www.w3.org/TR/2009/WD-eventsource-20091222/</a></dd>
<dd><a href="http://www.w3.org/TR/2009/WD-eventsource-20091029/">http://www.w3.org/TR/2009/WD-eventsource-20091029/</a></dd>
<dd><a href="http://www.w3.org/TR/2009/WD-eventsource-20090423/">http://www.w3.org/TR/2009/WD-eventsource-20090423/</a></dd>
<!-- :ZZZ -->
<dt>Editor:</dt>
<dd><a href="mailto:ian@hixie.ch">Ian Hickson</a>, Google, Inc.</dd>
</dl><p class="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a>
© 2011 <a href="http://www.w3.org/"><abbr title="World Wide
Web Consortium">W3C</abbr></a><sup>®</sup> (<a href="http://www.csail.mit.edu/"><abbr title="Massachusetts
Institute of Technology">MIT</abbr></a>, <a href="http://www.ercim.eu/"><abbr title="European Research
Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C
<a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
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<p class="alt">The bulk of the text of this specification is also
available in the WHATWG <a href="http://www.whatwg.org/specs/web-apps/current-work/complete.html#eventsource">Web Applications 1.0</a> specification, under a license that permits
reuse of the specification text.</p>
<!-- UNDER NO CIRCUMSTANCES IS THE PRECEDING PARAGRAPH TO BE REMOVED OR EDITED WITHOUT TALKING TO IAN FIRST -->
</div><hr class="top"><h2 class="no-num no-toc" id="abstract">Abstract</h2><p>This specification defines an API for opening an HTTP connection
for receiving push notifications from a server in the form of DOM
events. The API is designed such that it can be extended to work
with other push notification schemes such as Push SMS.<h2 class="no-num no-toc" id="status-of-this-document">Status of This document</h2><p><em>This section describes the status of this document at the
time of its publication. Other documents may supersede this
document. A list of current W3C publications and the
latest <!-- DO NOT CHANGE THIS BACK TO THE STANDARD BOILERPLATE, AS IT IS INACCURATE -->
revision of this technical report can be found in the <a href="http://www.w3.org/TR/">W3C technical reports index</a> at
http://www.w3.org/TR/.</em></p><p>If you wish to make comments regarding this document in a manner
that is tracked by the W3C, please submit them via using <a href="http://www.w3.org/Bugs/Public/enter_bug.cgi?product=HTML%20WG">our
public bug database</a>. If you do not have an account then you can
enter feedback using this form:<form action="http://www.whatwg.org/specs/web-apps/current-work/file-spam.cgi" method="post">
<fieldset><legend>Feedback Comments</legend>
<input name="id" type="hidden" value="top"><input name="component" type="hidden" value="Server-Sent Events (editor: Ian Hickson)"><input name="response" type="hidden" value="html"><p><label for="feedbackBox">Please enter your feedback, carefully
indicating the title of the section for which you are submitting
feedback, quoting the text that's wrong today if appropriate. If
you're suggesting a new feature, it's really important to say
<em>what</em> the problem you're trying to solve is. That's more
important than the solution, in fact.</label></p>
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<p class="note">Please don't use section numbers as these tend to
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</fieldset></form><p>You can also e-mail feedback to <a href="mailto:public-webapps@w3.org">public-webapps@w3.org</a> (<a href="mailto:public-webapps-request@w3.org?subject=subscribe">subscribe</a>,
<a href="http://lists.w3.org/Archives/Public/public-webapps/">archives</a>),
or <a href="mailto:whatwg@whatwg.org">whatwg@whatwg.org</a> (<a href="http://lists.whatwg.org/listinfo.cgi/whatwg-whatwg.org">subscribe</a>,
<a href="http://lists.whatwg.org/pipermail/whatwg-whatwg.org/">archives</a>).
All feedback is welcome.</p><p>Implementors should be aware that this specification is not
stable. <strong>Implementors who are not taking part in the
discussions are likely to find the specification changing out from
under them in incompatible ways.</strong> Vendors interested in
implementing this specification before it eventually reaches the
Candidate Recommendation stage should join the aforementioned
mailing lists and take part in the discussions.<div id="multipage-common">
</div><p>The latest
stable version of the editor's draft of this specification is always
available on <a href="http://dev.w3.org/html5/eventsource/">the W3C CVS server</a>
and in the <a href="http://svn.whatwg.org/webapps/">WHATWG
Subversion repository</a>. The <a href="http://www.whatwg.org/specs/web-apps/current-work/complete.html">latest
editor's working copy</a> (which may contain unfinished text in the
process of being prepared) contains the latest draft text of this
specification (amongst others). For more details, please see the <a href="http://wiki.whatwg.org/wiki/FAQ#What_are_the_various_versions_of_the_spec.3F">WHATWG
FAQ</a>.<p>Notifications of changes to this specification are sent along
with notifications of changes to related specifications using the
following mechanisms:<dl><dt>E-mail notifications of changes</dt>
<dd>Commit-Watchers mailing list (complete source diffs): <a href="http://lists.whatwg.org/listinfo.cgi/commit-watchers-whatwg.org">http://lists.whatwg.org/listinfo.cgi/commit-watchers-whatwg.org</a></dd>
<dt>Browsable version-control record of all changes:</dt>
<dd>CVSWeb interface with side-by-side diffs: <a href="http://dev.w3.org/cvsweb/html5/">http://dev.w3.org/cvsweb/html5/</a></dd>
<dd>Annotated summary with unified diffs: <a href="http://html5.org/tools/web-apps-tracker">http://html5.org/tools/web-apps-tracker</a></dd>
<dd>Raw Subversion interface: <code>svn checkout http://svn.whatwg.org/webapps/</code></dd>
</dl><p>The W3C <a href="http://www.w3.org/2008/webapps/">Web Applications
Working Group</a> is the W3C working group responsible for this
specification's progress along the W3C Recommendation track.
This specification is the 20 October 2011 Working Draft.
</p><p>Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.</p><p>This document was produced by a group operating under the <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5
February 2004 W3C Patent Policy</a>. W3C maintains a <a href="http://www.w3.org/2004/01/pp-impl/42538/status" rel="disclosure">public list of
any patent disclosures</a> made in connection with the deliverables
of the group; that page also includes instructions for disclosing a
patent. An individual who has actual knowledge of a patent which the
individual believes contains <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section
6 of the W3C Patent Policy</a>.<h2 class="no-num no-toc" id="contents">Table of Contents</h2>
<ol class="toc">
<li><a href="#server-sent-events-intro"><span class="secno">1 </span>Introduction</a></li>
<li><a href="#conformance-requirements"><span class="secno">2 </span>Conformance requirements</a>
<ol>
<li><a href="#dependencies"><span class="secno">2.1 </span>Dependencies</a></ol></li>
<li><a href="#terminology"><span class="secno">3 </span>Terminology</a></li>
<li><a href="#the-eventsource-interface"><span class="secno">4 </span>The <code>EventSource</code> interface</a></li>
<li><a href="#processing-model"><span class="secno">5 </span>Processing model</a></li>
<li><a href="#parsing-an-event-stream"><span class="secno">6 </span>Parsing an event stream</a></li>
<li><a href="#event-stream-interpretation"><span class="secno">7 </span>Interpreting an event stream</a></li>
<li><a href="#notes"><span class="secno">8 </span>Notes</a></li>
<li><a href="#eventsource-push"><span class="secno">9 </span>Connectionless push and other features</a></li>
<li><a href="#garbage-collection"><span class="secno">10 </span>Garbage collection</a></li>
<li><a href="#iana-considerations"><span class="secno">11 </span>IANA considerations</a>
<ol>
<li><a href="#text-event-stream"><span class="secno">11.1 </span><code>text/event-stream</code></a></li>
<li><a href="#last-event-id"><span class="secno">11.2 </span><code>Last-Event-ID</code></a></ol></li>
<li><a class="no-num" href="#references">References</a></li>
<li><a class="no-num" href="#acknowledgements">Acknowledgements</a></ol>
<hr><h2 id="server-sent-events-intro"><span class="secno">1 </span>Introduction</h2><p><i>This section is non-normative.</i><p>To enable servers to push data to Web pages over HTTP or using
dedicated server-push protocols, this specification introduces the
<code><a href="#eventsource">EventSource</a></code> interface.<p>Using this API consists of creating an <code><a href="#eventsource">EventSource</a></code>
object and registering an event listener.<pre>var source = new EventSource('updates.cgi');
source.onmessage = function (event) {
alert(event.data);
};</pre><p>On the server-side, the script ("<code title="">updates.cgi</code>" in this case) sends messages in the
following form, with the <code><a href="#text-event-stream">text/event-stream</a></code> MIME
type:<pre>data: This is the first message.
data: This is the second message, it
data: has two lines.
data: This is the third message.</pre><hr><p>Authors can separate events by using different event types. Here
is a stream that has two event types, "add" and "remove":<pre>event: add
data: 73857293
event: remove
data: 2153
event: add
data: 113411</pre><p>The script to handle such a stream would look like this (where
<code title="">addHandler</code> and <code title="">removeHandler</code> are functions that take one argument,
the event):<pre>var source = new EventSource('updates.cgi');
source.addEventListener('add', addHandler, false);
source.addEventListener('remove', removeHandler, false);</pre><p>The default event type is "message".<hr><p>Event streams requests can be redirected using HTTP 301 and 307
redirects as with normal HTTP requests. Clients will reconnect if
the connection is closed; a client can be told to stop reconnecting
using the HTTP 204 No Content response code.<p>Using this API rather than emulating it using
<code>XMLHttpRequest</code> or an <code>iframe</code> allows the
user agent to make better use of network resources in cases where
the user agent implementor and the network operator are able to
coordinate in advance. Amongst other benefits, this can result in
significant savings in battery life on portable devices. This is
discussed further in the section below on <a href="#eventsource-push">connectionless push</a>.<h2 id="conformance-requirements"><span class="secno">2 </span>Conformance requirements</h2><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>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="#refsRFC2119">[RFC2119]</a><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>Some conformance requirements are phrased as requirements on
attributes, methods or objects. Such requirements are to be
interpreted as requirements on user agents.<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>The only conformance class defined by this specification is user
agents.<p>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>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 — leaving the attribute on the object but
making it return null or throw an exception is insufficient.<h3 id="dependencies"><span class="secno">2.1 </span>Dependencies</h3><p>This specification relies on several other underlying
specifications.<dl><dt>HTML</dt>
<dd>
<p>Many fundamental concepts from HTML are used by this
specification. <a href="#refsHTML">[HTML]</a></p>
</dd>
<dt>WebIDL</dt>
<dd>
<p>The IDL blocks in this specification use the semantics of the
WebIDL specification. <a href="#refsWEBIDL">[WEBIDL]</a></p>
</dd>
</dl><h2 id="terminology"><span class="secno">3 </span>Terminology</h2><p>The construction "a <code title="">Foo</code> object", where
<code title="">Foo</code> is actually an interface, is sometimes
used instead of the more accurate "an object implementing the
interface <code title="">Foo</code>".<p>The term DOM is used to refer to the API set made available to
scripts in Web applications, and does not necessarily imply the
existence of an actual <code>Document</code> object or of any other
<code>Node</code> objects as defined in the DOM Core
specifications. <a href="#refsDOMCORE">[DOMCORE]</a><p>An IDL attribute is said to be <em>getting</em> when its value is
being retrieved (e.g. by author script), and is said to be
<em>setting</em> when a new value is assigned to it.<h2 id="the-eventsource-interface"><span class="secno">4 </span>The <code><a href="#eventsource">EventSource</a></code> interface</h2><pre class="idl">[<a href="#dom-eventsource" title="dom-EventSource">Constructor</a>(DOMString url)]
interface <dfn id="eventsource">EventSource</dfn> : <span>EventTarget</span> {
readonly attribute DOMString <a href="#dom-eventsource-url" title="dom-EventSource-url">url</a>;
// ready state
const unsigned short <a href="#dom-eventsource-connecting" title="dom-EventSource-CONNECTING">CONNECTING</a> = 0;
const unsigned short <a href="#dom-eventsource-open" title="dom-EventSource-OPEN">OPEN</a> = 1;
const unsigned short <a href="#dom-eventsource-closed" title="dom-EventSource-CLOSED">CLOSED</a> = 2;
readonly attribute unsigned short <a href="#dom-eventsource-readystate" title="dom-EventSource-readyState">readyState</a>;
// networking
[TreatNonCallableAsNull] attribute <span>Function</span>? <a href="#handler-eventsource-onopen" title="handler-EventSource-onopen">onopen</a>;
[TreatNonCallableAsNull] attribute <span>Function</span>? <a href="#handler-eventsource-onmessage" title="handler-EventSource-onmessage">onmessage</a>;
[TreatNonCallableAsNull] attribute <span>Function</span>? <a href="#handler-eventsource-onerror" title="handler-EventSource-onerror">onerror</a>;
void <a href="#dom-eventsource-close" title="dom-EventSource-close">close</a>();
};</pre><p>The <dfn id="dom-eventsource" title="dom-EventSource"><code>EventSource(<var title="">url</var>)</code></dfn> constructor takes one argument,
<var title="">url</var>, which specifies the <span>URL</span> to
which to connect. When the <code>EventSource()</code> constructor is
invoked, the UA must run these steps:<ol><li><p><span title="resolve a url">Resolve</span> the
<span>URL</span> specified in <var title="">url</var>, relative to
the <span>entry script</span>'s <span title="script's base
URL">base URL</span>.</li>
<li><p>If the previous step failed, then throw a
<code>SyntaxError</code> exception.</li>
<li><p>Return a new <code><a href="#eventsource">EventSource</a></code> object, and continue
these steps in the background (without blocking scripts).</li>
<li>
<p>Do a <span>potentially CORS-enabled fetch</span> of the
resulting <span>absolute URL</span>, with the <i>mode</i> being
<span title="attr-crossorigin-use-credentials">Use
Credentials</span>, and the <i title="">origin</i> being the <span>entry
script</span>'s <span>origin</span>, and process the resource obtained in
this fashion, if any, as described below.</p>
<p class="note">The definition of the <span title="fetch">fetching</span> algorithm (which is used by CORS) is
such that if the browser is already fetching the resource
identified by the given <span>absolute URL</span>, that connection
can be reused, instead of a new connection being established. All
messages received up to this point are dispatched immediately, in
this case.</p>
</li>
</ol><p>This constructor must be visible when the <span>script's global
object</span> is either a <code>Window</code> object or an object
implementing the <code>WorkerUtils</code> interface.<hr><p>The <dfn id="dom-eventsource-url" title="dom-EventSource-url"><code>url</code></dfn>
attribute must return the <span>absolute URL</span> that resulted
from <span title="resolve a url">resolving</span> the value that was
passed to the constructor.</p><p>The <dfn id="dom-eventsource-readystate" title="dom-EventSource-readyState"><code>readyState</code></dfn>
attribute represents the state of the connection. It can have the
following values:<dl><dt><dfn id="dom-eventsource-connecting" title="dom-EventSource-CONNECTING"><code>CONNECTING</code></dfn> (numeric value 0)</dt>
<dd>The connection has not yet been established, or it was closed
and the user agent is reconnecting.</dd>
<dt><dfn id="dom-eventsource-open" title="dom-EventSource-OPEN"><code>OPEN</code></dfn> (numeric value 1)</dt>
<dd>The user agent has an open connection and is dispatching events
as it receives them.</dd>
<dt><dfn id="dom-eventsource-closed" title="dom-EventSource-CLOSED"><code>CLOSED</code></dfn> (numeric value 2)</dt>
<dd>The connection is not open, and the user agent is not trying to
reconnect. Either there was a fatal error or the <code title="dom-EventSource-close"><a href="#dom-eventsource-close">close()</a></code> method was
invoked.</dd>
</dl><p>When the object is created its <code title="dom-EventSource-readyState"><a href="#dom-eventsource-readystate">readyState</a></code> must be set to
<code title="dom-EventSource-CONNECTING"><a href="#dom-eventsource-connecting">CONNECTING</a></code> (0). The
rules given below for handling the connection define when the value
changes.<p>The <dfn id="dom-eventsource-close" title="dom-EventSource-close"><code>close()</code></dfn>
method must abort any instances of the <span>fetch</span> algorithm
started for this <code><a href="#eventsource">EventSource</a></code> object, and must set the
<code title="dom-EventSource-readyState"><a href="#dom-eventsource-readystate">readyState</a></code> attribute
to <code title="dom-EventSource-CLOSED"><a href="#dom-eventsource-closed">CLOSED</a></code>.</p><p>The following are the <span>event handlers</span> (and their
corresponding <span title="event handler event type">event handler
event types</span>) that must be supported, as IDL attributes, by
all objects implementing the <code><a href="#eventsource">EventSource</a></code> interface:<table><thead><tr><th><span title="event handlers">Event handler</span> <th><span>Event handler event type</span>
<tbody><tr><td><dfn id="handler-eventsource-onopen" title="handler-EventSource-onopen"><code>onopen</code></dfn> <td> <code title="event-open">open</code>
<tr><td><dfn id="handler-eventsource-onmessage" title="handler-EventSource-onmessage"><code>onmessage</code></dfn> <td> <code title="event-message">message</code>
<tr><td><dfn id="handler-eventsource-onerror" title="handler-EventSource-onerror"><code>onerror</code></dfn> <td> <code title="event-error">error</code>
</table><hr><p>In addition to the above, each <code><a href="#eventsource">EventSource</a></code> object
has the following associated with it:<ul><li>A <dfn id="concept-event-stream-reconnection-time" title="concept-event-stream-reconnection-time">reconnection
time</dfn>, in milliseconds. This must initially be a
user-agent-defined value, probably in the region of a few
seconds.</li>
<li>A <dfn id="concept-event-stream-last-event-id" title="concept-event-stream-last-event-id">last event
ID string</dfn>. This must initially be the empty string.</li>
</ul><p>These values are not currently exposed on the interface.<h2 id="processing-model"><span class="secno">5 </span>Processing model</h2><p>The resource indicated in the argument to the <code title="dom-EventSource"><a href="#dom-eventsource">EventSource</a></code> constructor is <span title="fetch">fetched</span> when the constructor is run.<p>For HTTP connections, the <code title="">Accept</code> header may
be included; if included, it must contain only formats of event
framing that are supported by the user agent (one of which must be
<code><a href="#text-event-stream">text/event-stream</a></code>, as described below).<p>If the event source's <a href="#concept-event-stream-last-event-id" title="concept-event-stream-last-event-id">last event ID
string</a> is not the empty string, then a <code title="http-last-event-id"><a href="#last-event-id">Last-Event-ID</a></code> HTTP header must be
included with the request, whose value is the value of the event
source's <a href="#concept-event-stream-last-event-id" title="concept-event-stream-last-event-id">last event
ID string</a>, encoded as UTF-8.<p>User agents should use the <code>Cache-Control: no-cache</code>
header in requests to bypass any caches for requests of event
sources. (This header is not a <span title="custom request
headers">custom request header</span>, so the user agent will still
use the CORS <span>simple cross-origin request</span> mechanism.)
User agents should ignore HTTP cache headers in the response, never
caching event sources.<hr><p>As data is received, the <span title="concept-task">tasks</span>
queued by the <span>networking task source</span> to handle the data
must act as follows.<p>HTTP 200 OK responses with a <span>Content-Type</span> header
specifying the type <code><a href="#text-event-stream">text/event-stream</a></code>, either with no
parameters or with a single parameter with the name "<code title="">charset</code>" whose value is an <span>ASCII
case-insensitive</span> match for the string "<code title="">utf-8</code>", must be processed line by line <a href="#event-stream-interpretation">as described below</a>.<p>When a successful response with a supported <span>MIME
type</span> is received, such that the user agent begins parsing the
contents of the stream, the user agent must <a href="#announce-the-connection">announce the
connection</a>.<p>The <span title="concept-task">task</span> that the
<span>networking task source</span> places on the <span>task
queue</span> once the <span title="fetch">fetching algorithm</span>
for such a resource (with the correct <span>MIME type</span>) has
completed must <a href="#reestablish-the-connection">reestablish the connection</a>. This applies
whether the connection is closed gracefully or unexpectedly. It
doesn't apply for the error conditions listed below.<p>HTTP 200 OK responses that have a <span>Content-Type</span>
specifying an unsupported type (including the
<code><a href="#text-event-stream">text/event-stream</a></code> type with unsupported parameters or
parameters with unsupported values), or that have no
<span>Content-Type</span> at all, must cause the user agent to
<a href="#fail-the-connection">fail the connection</a>.</p><p>HTTP 305 Use Proxy, HTTP 401 Unauthorized, and 407 Proxy
Authentication Required should be treated transparently as for any
other subresource.<p>HTTP 301 Moved Permanently, HTTP 302 Found, 303 See Other, and
307 Temporary Redirect responses are handled by the <span title="fetch">fetching</span> and CORS algorithms. In the case of
301 redirects, the user agent must also remember the new URL so that
subsequent requests for this resource for this
<code><a href="#eventsource">EventSource</a></code> object start with the URL given for the
last 301 seen for requests for this object.<p>Any other HTTP response code not listed here, and any network
error that prevents the HTTP connection from being established in
the first place (e.g. DNS errors), must cause the user agent to
<a href="#fail-the-connection">fail the connection</a>.</p><p>For non-HTTP protocols, UAs should act in equivalent ways.<hr><p>When a user agent is to <dfn id="announce-the-connection">announce the connection</dfn>, the
user agent must <span>queue a task</span> to set the <code title="dom-EventSource-readyState"><a href="#dom-eventsource-readystate">readyState</a></code> attribute to
<code title="dom-EventSource-OPEN"><a href="#dom-eventsource-open">OPEN</a></code> and <span>fire a
simple event</span> named <code title="event-open">open</code> at
the <code><a href="#eventsource">EventSource</a></code> object.<p>When a user agent is to <dfn id="reestablish-the-connection">reestablish the connection</dfn>,
the user agent must <span>queue a task</span> to set the <code title="dom-EventSource-readyState"><a href="#dom-eventsource-readystate">readyState</a></code> attribute to
<code title="dom-EventSource-CONNECTING"><a href="#dom-eventsource-connecting">CONNECTING</a></code> and
<span>fire a simple event</span> named <code title="event-error">error</code> at the <code><a href="#eventsource">EventSource</a></code>
object, and then, after a delay equal to the reconnection time of
the event source, if the <code title="dom-EventSource-readyState"><a href="#dom-eventsource-readystate">readyState</a></code> attribute is
still set to <code title="dom-EventSource-CONNECTING"><a href="#dom-eventsource-connecting">CONNECTING</a></code>, once again do
a <span>potentially CORS-enabled fetch</span> of the <span>absolute
URL</span> of the event source resource, with the <i>mode</i> being
<span title="attr-crossorigin-use-credentials">Use
Credentials</span>, and the <i title="">origin</i> being the same as the
<span>origin</span> used in the original request triggered by the
<code title="dom-EventSource"><a href="#dom-eventsource">EventSource()</a></code> constructor, and process the
resource obtained in this fashion, if any, as described in this
section.<p>When a user agent is to <dfn id="fail-the-connection">fail the connection</dfn>, the user
agent must <span>queue a task</span> to set the <code title="dom-EventSource-readyState"><a href="#dom-eventsource-readystate">readyState</a></code> attribute to
<code title="dom-EventSource-CLOSED"><a href="#dom-eventsource-closed">CLOSED</a></code> and <span>fire a
simple event</span> named <code title="event-error">error</code> at
the <code><a href="#eventsource">EventSource</a></code> object. <strong>Once the user agent has
<a href="#fail-the-connection" title="fail the connection">failed the connection</a>, it
does <em>not</em> attempt to reconnect!</strong><hr><p>The <span>task source</span> for any <span title="concept-task">tasks</span> that are <span title="queue a
task">queued</span> by <code><a href="#eventsource">EventSource</a></code> objects is the
<dfn id="remote-event-task-source">remote event task source</dfn>.<h2 id="parsing-an-event-stream"><span class="secno">6 </span>Parsing an event stream</h2><p>This event stream format's <span>MIME type</span> is
<code><a href="#text-event-stream">text/event-stream</a></code>.<p>The event stream format is as described by the <code title="">stream</code> production of the following ABNF, the
character set for which is Unicode. <a href="#refsABNF">[ABNF]</a><pre>stream = [ bom ] *event
event = *( comment / field ) end-of-line
comment = colon *any-char end-of-line
field = 1*name-char [ colon [ space ] *any-char ] end-of-line
end-of-line = ( cr lf / cr / lf )
; characters
lf = %x000A ; U+000A LINE FEED (LF)
cr = %x000D ; U+000D CARRIAGE RETURN (CR)
space = %x0020 ; U+0020 SPACE
colon = %x003A ; U+003A COLON (:)
bom = %xFEFF ; U+FEFF BYTE ORDER MARK
name-char = %x0000-0009 / %x000B-000C / %x000E-0039 / %x003B-10FFFF
; a <span>Unicode character</span> other than U+000A LINE FEED (LF), U+000D CARRIAGE RETURN (CR), or U+003A COLON (:)
any-char = %x0000-0009 / %x000B-000C / %x000E-10FFFF
; a <span>Unicode character</span> other than U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR)</pre><p>Event streams in this format must always be encoded as
UTF-8. <a href="#refsRFC3629">[RFC3629]</a><p>Lines must be separated by either a U+000D CARRIAGE RETURN U+000A
LINE FEED (CRLF) character pair, a single U+000A LINE FEED (LF)
character, or a single U+000D CARRIAGE RETURN (CR) character.<p>Since connections established to remote servers for such
resources are expected to be long-lived, UAs should ensure that
appropriate buffering is used. In particular, while line buffering
with lines are defined to end with a single U+000A LINE FEED (LF)
character is safe, block buffering or line buffering with different
expected line endings can cause delays in event dispatch.<h2 id="event-stream-interpretation"><span class="secno">7 </span>Interpreting an event stream</h2><p>Streams must be <span>decoded as UTF-8, with error
handling</span>.
<a href="#refsHTML">[HTML]</a>
<p>One leading U+FEFF BYTE ORDER MARK character must be ignored if
any are present.<p>The stream must then be parsed by reading everything line by
line, with a U+000D CARRIAGE RETURN U+000A LINE FEED (CRLF)
character pair, a single U+000A LINE FEED (LF) character not
preceded by a U+000D CARRIAGE RETURN (CR) character, a single U+000D
CARRIAGE RETURN (CR) character not followed by a U+000A LINE FEED
(LF) character, and the end of the file being the four ways in which
a line can end.<p>When a stream is parsed, a <var title="">data</var> buffer, an
<var title="">event name</var> buffer, and a <var title="">last
event ID</var> buffer must be associated with it. They must be
initialized to the empty string<p>Lines must be processed, in the order they are received, as
follows:<dl class="switch"><dt>If the line is empty (a blank line)</dt>
<dd><p><a href="#dispatchMessage">Dispatch the event</a>, as
defined below.</dd>
<dt>If the line starts with a U+003A COLON character (:)</dt>
<dd><p>Ignore the line.</dd>
<dt>If the line contains a U+003A COLON character (:)</dt>
<dd>
<p>Collect the characters on the line before the first U+003A
COLON character (:), and let <var title="">field</var> be that
string.</p>
<p>Collect the characters on the line after the first U+003A COLON
character (:), and let <var title="">value</var> be that
string. If <var title="">value</var> starts with a U+0020
SPACE character, remove it from <var title="">value</var>.</p>
<p><a href="#processField">Process the field</a> using the steps
described below, using <var title="">field</var> as the field name
and <var title="">value</var> as the field value.</p>
</dd>
<dt>Otherwise, the string is not empty but does not contain a U+003A COLON character (:)</dt>
<dd>
<p><a href="#processField">Process the field</a> using the steps
described below, using the whole line as the field name, and
the empty string as the field value.</p>
</dd>
</dl><p>Once the end of the file is reached, any pending data must be
discarded. (If the file ends in the middle of an event, before the
final empty line, the incomplete event is not dispatched.)<hr><p id="processField">The steps to <dfn title="">process the
field</dfn> given a field name and a field value depend on the field
name, as given in the following list. Field names must be compared
literally, with no case folding performed.<dl class="switch"><dt>If the field name is "event"</dt>
<dd><p>Set the <var title="">event name</var> buffer to field
value.</dd>
<dt>If the field name is "data"</dt>
<dd><p>Append the field value to the <var title="">data</var>
buffer, then append a single U+000A LINE FEED (LF) character to the
<var title="">data</var> buffer.</dd>
<dt>If the field name is "id"</dt>
<dd><p>Set the <var title="">last event ID</var> buffer to the
field value.</dd>
<dt>If the field name is "retry"</dt>
<dd><p>If the field value consists of only characters in the range
U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9), then interpret the
field value as an integer in base ten, and set the event stream's
<a href="#concept-event-stream-reconnection-time" title="concept-event-stream-reconnection-time">reconnection
time</a> to that integer. Otherwise, ignore the field.</dd>
<dt>Otherwise</dt>
<dd><p>The field is ignored.</dd>
</dl><p id="dispatchMessage">When the user agent is required to <dfn title="">dispatch the event</dfn>, then the user agent must act as
follows:
<ol><li><p>Set the <a href="#concept-event-stream-last-event-id" title="concept-event-stream-last-event-id">last event ID
string</a> of the event source to value of the <var title="">last event ID</var> buffer. The buffer does not get reset,
so the <a href="#concept-event-stream-last-event-id" title="concept-event-stream-last-event-id">last event
ID string</a> of the event source remains set to this value
until the next time it is set by the server.</li>
<li><p>If the <var title="">data</var> buffer is an empty string,
set the <var title="">data</var> buffer and the <var title="">event
name</var> buffer to the empty string and abort these
steps.</li>
<li><p>If the <var title="">data</var> buffer's last character is a
U+000A LINE FEED (LF) character, then remove the last character
from the <var title="">data</var> buffer.</li>
<li><p>Create an event that uses the <code>MessageEvent</code>
interface, with the event name <code title="event-message">message</code>, which does not bubble, is not
cancelable, and has no default action. The <code title="dom-MessageEvent-data">data</code> attribute must be
initialized to the value of the <var title="">data</var> buffer,
the <code title="dom-MessageEvent-origin">origin</code> attribute
must be initialized to the <span title="Unicode serialization of an
origin">Unicode serialization</span> of the <span>origin</span> of
the event stream's URL, and the <code title="dom-MessageEvent-lastEventId">lastEventId</code> attribute
must be initialized to the <a href="#concept-event-stream-last-event-id" title="concept-event-stream-last-event-id">last event ID
string</a> of the event source.</li>
<li><p>If the <var title="">event name</var> buffer has a value
other than the empty string, change the <span title="concept-event-type">type</span> of the newly created event
to equal the value of the <var title="">event name</var>
buffer.</li>
<li><p>Set the <var title="">data</var> buffer and the <var title="">event name</var> buffer to the empty string.</li>
<li><p><span>Queue a task</span> which, if the <code title="dom-EventSource-readyState"><a href="#dom-eventsource-readystate">readyState</a></code> attribute is
set to a value other than <code title="dom-EventSource-CLOSED"><a href="#dom-eventsource-closed">CLOSED</a></code>, dispatches the newly
created event at the <code><a href="#eventsource">EventSource</a></code> object.</li>
</ol><p class="note">If an event doesn't have an "id" field, but an
earlier event did set the event source's <a href="#concept-event-stream-last-event-id" title="concept-event-stream-last-event-id">last event ID
string</a>, then the event's <code title="dom-MessageEvent-lastEventId">lastEventId</code> field will
be set to the value of whatever the last seen "id" field was.<div class="example">
<p>The following event stream, once followed by a blank line:</p>
<pre>data: YHOO
data: +2
data: 10</pre>
<p>...would cause an event <code title="event-message">message</code> with the interface
<code>MessageEvent</code> to be dispatched on the
<code><a href="#eventsource">EventSource</a></code> object. The event's <code title="dom-MessageEvent-data">data</code> attribute would contain
the string <code>YHOO\n+2\n10</code> (where <code>\n</code>
represents a newline).</p>
<p>This could be used as follows:
<pre>var stocks = new EventSource("http://stocks.example.com/ticker.php");
stocks.onmessage = function (event) {
var data = event.data.split('\n');
updateStocks(data[0], data[1], data[2]);
};</pre>
<p>...where <code title="">updateStocks()</code> is a function defined as:</p>
<pre>function updateStocks(symbol, delta, value) { ... }</pre>
<p>...or some such.</p>
</div><div class="example">
<p>The following stream contains four blocks. The first block has
just a comment, and will fire nothing. The second block has two
fields with names "data" and "id" respectively; an event will be
fired for this block, with the data "first event", and will then
set the last event ID to "1" so that if the connection died between
this block and the next, the server would be sent a <code title="http-last-event-id"><a href="#last-event-id">Last-Event-ID</a></code> header with the
value "1". The third block fires an event with data "second event",
and also has an "id" field, this time with no value, which resets
the last event ID to the empty string (meaning no <code title="http-last-event-id"><a href="#last-event-id">Last-Event-ID</a></code> header will now be
sent in the event of a reconnection being attempted). Finally, the
last block just fires an event with the data
" third event" (with a single leading space character).
Note that the last still has to end with a blank line, the end of
the stream is not enough to trigger the dispatch of the last
event.</p>
<pre>: test stream
data: first event
id: 1
data:second event
id
data: third event
</pre>
</div><div class="example">
<p>The following stream fires two events:</p>
<pre>data
data
data
data:</pre>
<p>The first block fires events with the data set to the empty
string, as would the last block if it was followed by a blank line.
The middle block fires an event with the data set to a single
newline character. The last block is discarded because it is not
followed by a blank line.</p>
</div><div class="example">
<p>The following stream fires two identical events:</p>
<pre>data:test
data: test
</pre>
<p>This is because the space after the colon is ignored if
present.</p>
</div><h2 id="notes"><span class="secno">8 </span>Notes</h2><p>Legacy proxy servers are known to, in certain cases, drop HTTP
connections after a short timeout. To protect against such proxy
servers, authors can include a comment line (one starting with a ':'
character) every 15 seconds or so.<p>Authors wishing to relate event source connections to each other
or to specific documents previously served might find that relying
on IP addresses doesn't work, as individual clients can have
multiple IP addresses (due to having multiple proxy servers) and
individual IP addresses can have multiple clients (due to sharing a
proxy server). It is better to include a unique identifier in the
document when it is served and then pass that identifier as part of
the URL when the connection is established.<p>Authors are also cautioned that HTTP chunking can have unexpected
negative effects on the reliability of this protocol. Where
possible, chunking should be disabled for serving event streams
unless the rate of messages is high enough for this not to
matter.</p><p>Clients that support HTTP's per-server connection limitation
might run into trouble when opening multiple pages from a site if
each page has an <code><a href="#eventsource">EventSource</a></code> to the same
domain. Authors can avoid this using the relatively complex
mechanism of using unique domain names per connection, or by
allowing the user to enable or disable the <code><a href="#eventsource">EventSource</a></code>
functionality on a per-page basis, or by sharing a single
<code><a href="#eventsource">EventSource</a></code> object using a <span title="SharedWorkerGlobalScope">shared worker</span>.
<a href="#refsWEBWORKERS">[WEBWORKERS]</a>
<h2 id="eventsource-push"><span class="secno">9 </span>Connectionless push and other features</h2><p>User agents running in controlled environments, e.g. browsers on
mobile handsets tied to specific carriers, may offload the
management of the connection to a proxy on the network. In such a
situation, the user agent for the purposes of conformance is
considered to include both the handset software and the network
proxy.<div class="example">
<p>For example, a browser on a mobile device, after having
established a connection, might detect that it is on a supporting
network and request that a proxy server on the network take over
the management of the connection. The timeline for such a situation
might be as follows:</p>
<ol><li>Browser connects to a remote HTTP server and requests the
resource specified by the author in the <code title="dom-EventSource"><a href="#dom-eventsource">EventSource</a></code> constructor.</li>
<li>The server sends occasional messages.</li>
<li>In between two messages, the browser detects that it is idle
except for the network activity involved in keeping the TCP
connection alive, and decides to switch to sleep mode to save power.</li>
<li>The browser disconnects from the server.</li>
<li>The browser contacts a service on the network, and requests
that that service, a "push proxy", maintain the connection instead.</li>
<li>The "push proxy" service contacts the remote HTTP server and
requests the resource specified by the author in the <code title="dom-EventSource"><a href="#dom-eventsource">EventSource</a></code> constructor (possibly
including a <code title="http-last-event-id"><a href="#last-event-id">Last-Event-ID</a></code>
HTTP header, etc).</li>
<li>The browser allows the mobile device to go to sleep.</li>
<li>The server sends another message.</li>
<li>The "push proxy" service uses a technology such as OMA push to
convey the event to the mobile device, which wakes only
enough to process the event and then returns to sleep.</li>
</ol></div><p>This can reduce the total data usage, and can therefore result in
considerable power savings.<p>As well as implementing the existing API and
<code><a href="#text-event-stream">text/event-stream</a></code> wire format as defined by this
specification and in more distributed ways as described above,
formats of event framing defined by <span>other applicable
specifications</span> may be supported. This specification does not
define how they are to be parsed or processed.<h2 id="garbage-collection"><span class="secno">10 </span>Garbage collection</h2><p>While an <code><a href="#eventsource">EventSource</a></code> object's <code title="dom-EventSource-readyState"><a href="#dom-eventsource-readystate">readyState</a></code> is <code title="dom-EventSource-CONNECTING"><a href="#dom-eventsource-connecting">CONNECTING</a></code>, and the object
has one or more event listeners registered for <code title="event-open">open</code>, <code title="event-message">message</code> or <code title="event-error">error</code> events, there must be a strong
reference from the <code>Window</code> or <code>WorkerUtils</code>
object that the <code><a href="#eventsource">EventSource</a></code> object's constructor was
invoked from to the <code><a href="#eventsource">EventSource</a></code> object itself.<p>While an <code><a href="#eventsource">EventSource</a></code> object's <code title="dom-EventSource-readyState"><a href="#dom-eventsource-readystate">readyState</a></code> is <code title="dom-EventSource-OPEN"><a href="#dom-eventsource-open">OPEN</a></code>, and the object has one or
more event listeners registered for <code title="event-message">message</code> or <code title="event-error">error</code> events, there must be a strong
reference from the <code>Window</code> or <code>WorkerUtils</code>
object that the <code><a href="#eventsource">EventSource</a></code> object's constructor was
invoked from to the <code><a href="#eventsource">EventSource</a></code> object itself.<p>While there is a task queued by an <code><a href="#eventsource">EventSource</a></code>
object on the <a href="#remote-event-task-source">remote event task source</a>, there must be a
strong reference from the <code>Window</code> or
<code>WorkerUtils</code> object that the <code><a href="#eventsource">EventSource</a></code>
object's constructor was invoked from to that
<code><a href="#eventsource">EventSource</a></code> object.<p>If a user agent is to <dfn id="concept-eventsource-forcibly-close" title="concept-EventSource-forcibly-close">forcibly close</dfn> an
<code><a href="#eventsource">EventSource</a></code> object (this happens when a
<code>Document</code> object goes away permanently), the user agent
must abort any instances of the <span>fetch</span> algorithm started
for this <code><a href="#eventsource">EventSource</a></code> object, and must set the <code title="dom-EventSource-readyState"><a href="#dom-eventsource-readystate">readyState</a></code> attribute to
<code title="dom-EventSource-CLOSED"><a href="#dom-eventsource-closed">CLOSED</a></code>.</p><p>If an <code><a href="#eventsource">EventSource</a></code> object is garbage collected while
its connection is still open, the user agent must abort any instance
of the <span title="fetch">fetch</span> algorithm opened by this
<code><a href="#eventsource">EventSource</a></code>.</p><p class="note">It's possible for one active network connection to
be shared by multiple <code><a href="#eventsource">EventSource</a></code> objects and their
<span>fetch</span> algorithms, which is why the above is phrased in
terms of aborting the <span>fetch</span> algorithm and not the
actual underlying download.<h2 id="iana-considerations"><span class="secno">11 </span>IANA considerations</h2><h3 id="text-event-stream"><span class="secno">11.1 </span><dfn><code>text/event-stream</code></dfn></h3><p>This registration is for community review and will be submitted
to the IESG for review, approval, and registration with IANA.</p><dl><dt>Type name:</dt>
<dd>text</dd>
<dt>Subtype name:</dt>
<dd>event-stream</dd>
<dt>Required parameters:</dt>
<dd>No parameters</dd>
<dt>Optional parameters:</dt>
<dd>
<dl><dt><code title="">charset</code></dt>
<dd>
<p>The <code title="">charset</code> parameter may be provided.
The parameter's value must be "<code title="">utf-8</code>".
This parameter serves no purpose; it is only allowed for
compatibility with legacy servers.</p>
</dd>
</dl></dd>
<dt>Encoding considerations:</dt>
<dd>8bit (always UTF-8)</dd>
<dt>Security considerations:</dt>
<dd>
<p>An event stream from an origin distinct from the origin of the
content consuming the event stream can result in information
leakage. To avoid this, user agents are required to apply CORS
semantics. <a href="#refsCORS">[CORS]</a></p>
<p>Event streams can overwhelm a user agent; a user agent is
expected to apply suitable restrictions to avoid depleting local
resources because of an overabundance of information from an event
stream.</p>
<p>Servers can be overwhelmed if a situation develops in which the
server is causing clients to reconnect rapidly. Servers should use
a 5xx status code to indicate capacity problems, as this will
prevent conforming clients from reconnecting automatically.</p>
</dd>
<dt>Interoperability considerations:</dt>
<dd>
Rules for processing both conforming and non-conforming content
are defined in this specification.
</dd>
<dt>Published specification:</dt>
<dd>
This document is the relevant specification.
</dd>
<dt>Applications that use this media type:</dt>
<dd>
Web browsers and tools using Web services.
</dd>
<dt>Additional information:</dt>
<dd>
<dl><dt>Magic number(s):</dt>
<dd>No sequence of bytes can uniquely identify an event stream.</dd>
<dt>File extension(s):</dt>
<dd>No specific file extensions are recommended for this type.</dd>
<dt>Macintosh file type code(s):</dt>
<dd>No specific Macintosh file type codes are recommended for this type.</dd>
</dl></dd>
<dt>Person & email address to contact for further information:</dt>
<dd>Ian Hickson <ian@hixie.ch></dd>
<dt>Intended usage:</dt>
<dd>Common</dd>
<dt>Restrictions on usage:</dt>
<dd>This format is only expected to be used by dynamic open-ended
streams served using HTTP or a similar protocol. Finite resources
are not expected to be labeled with this type.</dd>
<dt>Author:</dt>
<dd>Ian Hickson <ian@hixie.ch></dd>
<dt>Change controller:</dt>
<dd>W3C</dd>
</dl><p>Fragment identifiers have no meaning with
<code><a href="#text-event-stream">text/event-stream</a></code> resources.<h3 id="last-event-id"><span class="secno">11.2 </span><dfn title="http-last-event-id"><code>Last-Event-ID</code></dfn></h3><p>This section describes a header field for registration in the
Permanent Message Header Field Registry. <a href="#refsRFC3864">[RFC3864]</a><dl><dt>Header field name</dt>
<dd>Last-Event-ID</dd>
<dt>Applicable protocol</dt>
<dd>http</dd>
<dt>Status</dt>
<dd>standard</dd>
<dt>Author/Change controller</dt>
<dd>W3C</dd>
<dt>Specification document(s)</dt>
<dd>
This document is the relevant specification.
</dd>
<dt>Related information</dt>
<dd>None.</dd>
</dl><h2 class="no-num" id="references">References</h2><p>All references are normative unless marked "Non-normative".</p><dl><dt id="refsABNF">[ABNF]</dt>
<dd><cite><a href="http://www.ietf.org/rfc/std/std68.txt">Augmented
BNF for Syntax Specifications: ABNF</a></cite>, D. Crocker,
P. Overell. IETF.</dd>
<dt id="refsCORS">[CORS]</dt>
<dd><cite><a href="http://dev.w3.org/2006/waf/access-control/">Cross-Origin
Resource Sharing</a></cite>, A. van Kesteren. W3C.</dd>
<dt id="refsDOMCORE">[DOMCORE]</dt>
<dd><cite><a href="http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html">Web DOM Core</a></cite>, A. van Kesteren. W3C.</dd>
<dt id="refsHTML">[HTML]</dt>
<dd><cite><a href="http://www.w3.org/TR/html5/">HTML5</a></cite>,
I. Hickson. W3C.</dd>
<dt id="refsRFC2119">[RFC2119]</dt>
<dd><cite><a href="http://tools.ietf.org/html/rfc2119">Key words for use in
RFCs to Indicate Requirement Levels</a></cite>, S. Bradner. IETF.</dd>
<dt id="refsRFC3629">[RFC3629]</dt>
<dd><cite><a href="http://tools.ietf.org/html/rfc3629">UTF-8, a
transformation format of ISO 10646</a></cite>, F. Yergeau. IETF.</dd>
<dt id="refsRFC3864">[RFC3864]</dt>
<dd><cite><a href="http://tools.ietf.org/html/rfc3864">Registration Procedures
for Message Header Fields</a></cite>, G. Klyne, M. Nottingham,
J. Mogul. IETF.</dd>
<dt id="refsWEBIDL">[WEBIDL]</dt>
<dd><cite><a href="http://dev.w3.org/2006/webapi/WebIDL/">Web
IDL</a></cite>, C. McCormack. W3C.</dd>
<dt id="refsWEBWORKERS">[WEBWORKERS]</dt>
<dd><cite><a href="http://dev.w3.org/html5/workers/">Web
Workers</a></cite>, I. Hickson. W3C.</dd>
</dl><h2 class="no-num" id="acknowledgements">Acknowledgements</h2><p>For a full list of acknowledgements, please see the HTML
specification. <a href="#refsHTML">[HTML]</a>
</body>
</html>