Allow external language information to apply

Make it possible for `<track srclang>` (or other external language)
to apply to the WebVTT nodes for the purpose of CSS `:lang()` matching,
glyph selection, etc.

"WebVTT cue text parsing rules" takes an optional fallback language.

The "rules for updating the display of WebVTT text tracks" also takes
an optional fallback language, since it is the entry point for HTML
and this algorithm then invokes the "WebVTT cue text parsing rules".

`getCueAsHTML()` does not provide a fallback language.

Fixes (1) in https://www.w3.org/Bugs/Public/show_bug.cgi?id=28255#c16.

index.bs

@@ -2517,7 +2517,7 @@ user agent must run the following steps:</p>
 </ol>
 
 
-<h3 id=cue-text-parsing-rules algorithm><dfn>WebVTT cue text parsing rules</dfn></h3>
+<h3 id=cue-text-parsing-rules algorithm>WebVTT cue text parsing rules</h3>
 
 <p>A <dfn>WebVTT Node Object</dfn> is a conceptual construct used to represent components of
 <a>WebVTT cue text</a> so that its processing can be described without reference to the underlying
@@ -2624,8 +2624,9 @@ Objects</a>:</p>
 
 </dl>
 
-<p>To parse a string |input| supposedly containing <a>WebVTT cue text</a>, user agents must use the
-following algorithm. This algorithm returns a <a>list of WebVTT Node Objects</a>.</p>
+<p>The <dfn>WebVTT cue text parsing rules</dfn> consist of the following algorithm. The input is a
+string |input| supposedly containing <a>WebVTT cue text</a>, and optionally a fallback language
+|language|. This algorithm returns a <a>list of WebVTT Node Objects</a>.</p>
 
 <ol>
 
@@ -2640,6 +2641,10 @@ following algorithm. This algorithm returns a <a>list of WebVTT Node Objects</a>
 
  <li><p>Let |language stack| be a stack of language tags, initially empty.</p></li>
 
+ <li><p>If |language| is set, set |result|'s <a lt="WebVTT Node Object's applicable
+ language">applicable language</a> to |language|, and push |language| onto the |language
+ stack|.</p></li>
+
  <li><p><i>Loop</i>: If |position| is past the end of |input|, return |result| and abort these
  steps.</p></li>
 
@@ -3345,7 +3350,8 @@ media players that don't support CSS, equivalent visual rendering will need to b
 track">text tracks</a> of a <a>media element</a> (specifically, a <a element>video</a> element), or
 of another playback mechanism, by applying the steps below. All the <a lt="text track">text
 tracks</a> that use these rules for a given <a>media element</a>, or other playback mechanism, are
-rendered together, to avoid overlapping subtitles from multiple tracks.</p>
+rendered together, to avoid overlapping subtitles from multiple tracks. A fallback language
+|language| may be set when calling this algorithm.</p>
 
 <p class="note">In HTML, audio elements don't have a visual rendering area and therefore, this
 algorithm will abort for audio elements. When authors do create WebVTT captions or subtitles for
@@ -3457,7 +3463,8 @@ manner suiting the user.</p>
   <ol>
 
    <li><p>Let |nodes| be the <a>list of WebVTT Node Objects</a> obtained by applying the <a>WebVTT
-   cue text parsing rules</a> to the |cue|'s <a>text track cue text</a>.</p></li>
+   cue text parsing rules</a>, with the fallback language |language| if provided, to the |cue|'s
+   <a>text track cue text</a>.</p></li>
 
    <li>
     <p>If |cue|'s <a>WebVTT cue region</a> is null, run the following substeps:</p>
@@ -4824,6 +4831,9 @@ spec=html>entry settings object</a> by applying the <a>WebVTT cue text DOM const
 the result of applying the <a>WebVTT cue text parsing rules</a> to the <a>text track cue
 text</a>.</p>
 
+<p class=note>A fallback language is not provided for {{VTTCue/getCueAsHTML()}} since a
+{{DocumentFragment}} cannot expose language information.</p>
+
 
 <h3 id=the-vttregion-interface algorithm>The {{VTTRegion}} interface</h3>
 

index.html

@@ -1111,7 +1111,7 @@ <h2 class="no-num no-toc no-ref heading settled" id="contents"><span class="cont
       <li><a href="#file-parsing"><span class="secno">5.1</span> <span class="content">WebVTT file parsing</span></a>
       <li><a href="#region-settings-parsing"><span class="secno">5.2</span> <span class="content">WebVTT region settings parsing</span></a>
       <li><a href="#cue-timings-and-settings-parsing"><span class="secno">5.3</span> <span class="content">WebVTT cue timings and settings parsing</span></a>
-      <li><a href="#cue-text-parsing-rules"><span class="secno">5.4</span> <span class="content"><span>WebVTT cue text parsing rules</span></span></a>
+      <li><a href="#cue-text-parsing-rules"><span class="secno">5.4</span> <span class="content">WebVTT cue text parsing rules</span></a>
       <li><a href="#dom-construction-rules"><span class="secno">5.5</span> <span class="content"><span>WebVTT cue text DOM construction rules</span></span></a>
       <li><a href="#rules-for-extracting-the-chapter-title"><span class="secno">5.6</span> <span class="content">WebVTT rules for extracting the chapter
 title</span></a>
@@ -2958,7 +2958,7 @@ <h3 class="heading settled" data-algorithm="WebVTT cue timings and settings pars
     <li>
      <p>Return <var>result</var>.</p>
    </ol>
-   <h3 class="heading settled" data-algorithm="WebVTT cue text parsing rules" data-level="5.4" id="cue-text-parsing-rules"><span class="secno">5.4. </span><span class="content"><dfn data-dfn-type="dfn" data-noexport="" id="webvtt-cue-text-parsing-rules">WebVTT cue text parsing rules<a class="self-link" href="#webvtt-cue-text-parsing-rules"></a></dfn></span><a class="self-link" href="#cue-text-parsing-rules"></a></h3>
+   <h3 class="heading settled" data-algorithm="WebVTT cue text parsing rules" data-level="5.4" id="cue-text-parsing-rules"><span class="secno">5.4. </span><span class="content">WebVTT cue text parsing rules</span><a class="self-link" href="#cue-text-parsing-rules"></a></h3>
    <p>A <dfn data-dfn-type="dfn" data-noexport="" id="webvtt-node-object">WebVTT Node Object<a class="self-link" href="#webvtt-node-object"></a></dfn> is a conceptual construct used to represent components of <a data-link-type="dfn" href="#webvtt-cue-text">WebVTT cue text</a> so that its processing can be described without reference to the underlying
 syntax.</p>
    <p>There are two broad classes of <a data-link-type="dfn" href="#webvtt-node-object">WebVTT Node Objects</a>: <a data-link-type="dfn" href="#webvtt-internal-node-object">WebVTT Internal Node Objects</a> and <a data-link-type="dfn" href="#webvtt-leaf-node-object">WebVTT
@@ -3025,8 +3025,8 @@ <h3 class="heading settled" data-algorithm="WebVTT cue text parsing rules" data-
      <p>A timestamp. A <a data-link-type="dfn" href="#webvtt-timestamp-object">WebVTT Timestamp Object</a> has a value, in seconds and fractions of a
   second, which is the time represented by the timestamp.</p>
    </dl>
-   <p>To parse a string <var>input</var> supposedly containing <a data-link-type="dfn" href="#webvtt-cue-text">WebVTT cue text</a>, user agents must use the
-following algorithm. This algorithm returns a <a data-link-type="dfn" href="#list-of-webvtt-node-objects">list of WebVTT Node Objects</a>.</p>
+   <p>The <dfn data-dfn-type="dfn" data-noexport="" id="webvtt-cue-text-parsing-rules">WebVTT cue text parsing rules<a class="self-link" href="#webvtt-cue-text-parsing-rules"></a></dfn> consist of the following algorithm. The input is a
+string <var>input</var> supposedly containing <a data-link-type="dfn" href="#webvtt-cue-text">WebVTT cue text</a>, and optionally a fallback language <var>language</var>. This algorithm returns a <a data-link-type="dfn" href="#list-of-webvtt-node-objects">list of WebVTT Node Objects</a>.</p>
    <ol>
     <li>
      <p>Let <var>input</var> be the string being parsed.</p>
@@ -3039,6 +3039,9 @@ <h3 class="heading settled" data-algorithm="WebVTT cue text parsing rules" data-
      <p>Let <var>current</var> be the <a data-link-type="dfn" href="#webvtt-internal-node-object">WebVTT Internal Node Object</a> <var>result</var>.</p>
     <li>
      <p>Let <var>language stack</var> be a stack of language tags, initially empty.</p>
+    <li>
+     <p>If <var>language</var> is set, set <var>result</var>'s <a data-link-type="dfn" href="#webvtt-node-objects-applicable-language">applicable language</a> to <var>language</var>, and push <var>language</var> onto the <var>language
+ stack</var>.</p>
     <li>
      <p><i>Loop</i>: If <var>position</var> is past the end of <var>input</var>, return <var>result</var> and abort these
  steps.</p>
@@ -3465,7 +3468,7 @@ <h3 class="heading settled" data-algorithm="Processing model" data-level="6.1" i
    <p>The <dfn data-dfn-type="dfn" data-noexport="" id="rules-for-updating-the-display-of-webvtt-text-tracks">rules for updating the display of WebVTT text tracks<a class="self-link" href="#rules-for-updating-the-display-of-webvtt-text-tracks"></a></dfn> render the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/embedded-content.html#text-track">text tracks</a> of a <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/embedded-content.html#media-element">media element</a> (specifically, a <a data-link-type="element" href="https://html.spec.whatwg.org/multipage/embedded-content.html#video">video</a> element), or
 of another playback mechanism, by applying the steps below. All the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/embedded-content.html#text-track">text
 tracks</a> that use these rules for a given <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/embedded-content.html#media-element">media element</a>, or other playback mechanism, are
-rendered together, to avoid overlapping subtitles from multiple tracks.</p>
+rendered together, to avoid overlapping subtitles from multiple tracks. A fallback language <var>language</var> may be set when calling this algorithm.</p>
    <p class="note" role="note">In HTML, audio elements don’t have a visual rendering area and therefore, this
 algorithm will abort for audio elements. When authors do create WebVTT captions or subtitles for
 audio resources, they need to publish them in a video element for rendering by the user agent.</p>
@@ -3551,7 +3554,7 @@ <h3 class="heading settled" data-algorithm="Processing model" data-level="6.1" i
      <ol>
       <li>
        <p>Let <var>nodes</var> be the <a data-link-type="dfn" href="#list-of-webvtt-node-objects">list of WebVTT Node Objects</a> obtained by applying the <a data-link-type="dfn" href="#webvtt-cue-text-parsing-rules">WebVTT
-   cue text parsing rules</a> to the <var>cue</var>'s <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/embedded-content.html#text-track-cue-text">text track cue text</a>.</p>
+   cue text parsing rules</a>, with the fallback language <var>language</var> if provided, to the <var>cue</var>'s <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/embedded-content.html#text-track-cue-text">text track cue text</a>.</p>
       <li>
        <p>If <var>cue</var>'s <a data-link-type="dfn" href="#webvtt-cue-region">WebVTT cue region</a> is null, run the following substeps:</p>
        <ol>
@@ -4473,6 +4476,7 @@ <h3 class="heading settled" data-algorithm="The VTTCue interface" data-level="7.
 text</a> to a <code class="idl"><a data-link-type="idl" href="https://dom.spec.whatwg.org/#documentfragment">DocumentFragment</a></code> for the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#responsible-document">responsible document</a> specified by the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#entry-settings-object">entry settings object</a> by applying the <a data-link-type="dfn" href="#webvtt-cue-text-dom-construction-rules">WebVTT cue text DOM construction rules</a> to
 the result of applying the <a data-link-type="dfn" href="#webvtt-cue-text-parsing-rules">WebVTT cue text parsing rules</a> to the <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/embedded-content.html#text-track-cue-text">text track cue
 text</a>.</p>
+   <p class="note" role="note">A fallback language is not provided for <code class="idl"><a data-link-type="idl" href="#dom-vttcue-getcueashtml">getCueAsHTML()</a></code> since a <code class="idl"><a data-link-type="idl" href="https://dom.spec.whatwg.org/#documentfragment">DocumentFragment</a></code> cannot expose language information.</p>
    <h3 class="heading settled" data-algorithm="The VTTRegion interface" data-level="7.2" id="the-vttregion-interface"><span class="secno">7.2. </span><span class="content">The <code class="idl"><a data-link-type="idl" href="#vttregion">VTTRegion</a></code> interface</span><a class="self-link" href="#the-vttregion-interface"></a></h3>
    <p>The following interface is used to expose WebVTT regions in the DOM API:</p>
 <pre class="idl">enum <dfn class="idl-code" data-dfn-type="enum" data-export="" id="enumdef-scrollsetting">ScrollSetting<a class="self-link" href="#enumdef-scrollsetting"></a></dfn> { "" /* none */, "up" };