Profile identifier and TTML registry

[cconcolato]

The profile registry for TTML is: https://www.w3.org/TR/ttml-profile-registry/
We need to have a profile identifier for DAPT. I would suggest ... dapt.

[cconcolato]

We may want to differentiate document types, e.g. those that require audio description mixing vs those that are text only. We could use dapa (audio needed) and dapt (text only).

[nigelmegitt]

We may want to differentiate document types, e.g. those that require audio description mixing vs those that are text only. We could use dapa (audio needed) and dapt (text only).

I would prefer not to - and we don't have distinct profiles for the two cases, so I'm not sure what we'd reference if we did do that.

[nigelmegitt]

Raised as w3c/tt-profile-registry#86