Add wrapping <dl> to the lineAlign steps

This was invalid markup, introduced in
w3c#116

webvtt.html

@@ -4324,38 +4324,40 @@ <h4>Processing model</h4>
           <li>
            <p>Run the appropriate steps from the following list:</p>
 
-           <dt>If the <a>text track cue writing direction</a> is <a title="text track cue horizontal
-           writing direction">horizontal</a></dt>
-           <dd>
-            <dl class="switch">
-             <dt>If the <a>text track cue line alignment</a> is <a title="text track cue text
-             position middle alignment">middle alignment</a></dt>
-             <dd><p>Move all the boxes in <var>boxes</var> up by half of the height of <var>bounding
-             box</var>.</p></dd>
-
-             <dt>If the <a>text track cue line alignment</a> is <a title="text track cue text
-             position end alignment">end alignment</a></dt>
-             <dd><p>Move all the boxes in <var>boxes</var> up by the height of <var>bounding
-             box</var>.</p></dd>
-            </dl>
-           </dd>
-
-           <dt>If the <a>text track cue writing direction</a> is <a title="text track cue vertical
-           growing left writing direction">vertical growing left</a> or <a title="text track cue
-           vertical growing right writing direction">vertical growing right</a></dt>
-           <dd>
-            <dl class="switch">
-             <dt>If the <a>text track cue line alignment</a> is <a title="text track cue text
-             position middle alignment">middle alignment</a></dt>
-             <dd><p>Move all the boxes in <var>boxes</var> left by half of the width of
-             <var>bounding box</var>.</p></dd>
-
-             <dt>If the <a>text track cue line alignment</a> is <a title="text track cue text
-             position end alignment">end alignment</a></dt>
-             <dd><p>Move all the boxes in <var>boxes</var> left by the width of <var>bounding
-             box</var>.</p></dd>
-            </dl>
-           </dd>
+           <dl class="switch">
+            <dt>If the <a>text track cue writing direction</a> is <a title="text track cue
+            horizontal writing direction">horizontal</a></dt>
+            <dd>
+             <dl class="switch">
+              <dt>If the <a>text track cue line alignment</a> is <a title="text track cue text
+              position middle alignment">middle alignment</a></dt>
+              <dd><p>Move all the boxes in <var>boxes</var> up by half of the height of
+              <var>bounding box</var>.</p></dd>
+
+              <dt>If the <a>text track cue line alignment</a> is <a title="text track cue text
+              position end alignment">end alignment</a></dt>
+              <dd><p>Move all the boxes in <var>boxes</var> up by the height of <var>bounding
+              box</var>.</p></dd>
+             </dl>
+            </dd>
+
+            <dt>If the <a>text track cue writing direction</a> is <a title="text track cue vertical
+            growing left writing direction">vertical growing left</a> or <a title="text track cue
+            vertical growing right writing direction">vertical growing right</a></dt>
+            <dd>
+             <dl class="switch">
+              <dt>If the <a>text track cue line alignment</a> is <a title="text track cue text
+              position middle alignment">middle alignment</a></dt>
+              <dd><p>Move all the boxes in <var>boxes</var> left by half of the width of
+              <var>bounding box</var>.</p></dd>
+
+              <dt>If the <a>text track cue line alignment</a> is <a title="text track cue text
+              position end alignment">end alignment</a></dt>
+              <dd><p>Move all the boxes in <var>boxes</var> left by the width of <var>bounding
+              box</var>.</p></dd>
+             </dl>
+            </dd>
+           </dl>
           </li>
 
           <li><p>If none of the boxes in <var>boxes</var> would overlap any of the boxes in