[ttml2] Pull Request: Mark #region-timing as optional (#767).

skynavga has just submitted a new pull request for https://github.com/w3c/ttml2:

== Mark #region-timing as optional (#767). ==
Closes #767.

Marking as editorial since profile def docs already mark as optional.

See https://github.com/w3c/ttml2/pull/775

Received on Friday, 18 May 2018 21:09:52 UTC