Specify or describe ttm:role attribute inheritance model

[nigelmegitt]

See also w3c/ttml2#1271 - I believe that the inheritance model of ttm:role is additive, but this is slightly unclear in TTML2. Suggest either explaining it in DAPT as a service to implementers, and possibly also gap-filling missing specification text in TTML2.

Specifically, in a case like:

<div ttm:role="action x-some-role-1">
  <p ttm:role="x-some-role-2">
    <!-- this p should end up with computed role of {"action", "x-some-role-1", "x-some-role-2"} -->
  </p>
</div>

It's easy to assume (as I did at first) that specifying a ttm:role attribute replaces any inherited role value, but there's no spec text to support that in TTML2; to the contrary, there is text that suggests that metadata, including role, specified on a content element applies to all of its descendant elements too.

I say "suggests" because there seems to be some vagueness in the case of metadata attributes on content elements, as raised in w3c/ttml2#1271.

[nigelmegitt]

w3c/ttml2#1271 has been resolved, clarifying that metadata attributes on content elements are indeed treated the same way as metadata specified by child metadata elements.