TTML

Yelp supports TTML subtitles in Mallard audio and video block media elements. To add subtitles, include a tt:tt element as a child of the media element.

Example from the Tetravex help:
<tt:tt xmlns:tt="http://www.w3.org/ns/ttml">
  <tt:body>
    <tt:div begin="1s" end="6s">
      <tt:p>Drag pieces from the right to the left, making sure that                                  
      adjacent edges have the same number and color.</tt:p>
    </tt:div>
    <tt:div begin="6s" end="14s">
      <tt:p>Hold down <key>Ctrl</key> and press the arrow keys to move                                
      all placed pieces at once.</tt:p>
    </tt:div>
    <tt:div begin="14s" end="22s">
      <tt:p>Continue dragging pieces until they all fit together on the left.</tt:p>
    </tt:div>
  </tt:body>
</tt:tt>

Begin and end times can be specified on tt:div, tt:p, and tt:span elements. In standard TTML, a tt:div element can contain other tt:div elements and tt:p elements, and a tt:p element can contain text, tt:span elements, and tt:br elements. Time ranges on child elements are relative to time ranges on parent elements.

Mallard extends TTML to allow you to use any block elements in a tt:div element and any inline elements in a tt:p or tt:span element. For example, in the TTML above, the Mallard key element is used inline in a tt:p element.

Basic support added in 3.0. Support for the dur attribute, the tt:br element, the h and m time units, and checking profiles added in 3.4. Support for TTML for audio elements added in 3.8.