Re: [w3ctag/design-reviews] Timed Text Markup Language 2 (TTML2) (#138)

Sorry for the long pause. https://github.com/w3c/ttml2/issues/406 is an umbrella issue for adding the CSS properties equivalent to TTML2 style attributes. They're all present in https://w3c.github.io/ttml2/#style-attribute-derivation

The current set of relevant open CSS issues for dealing with things that cannot be mapped is:

* https://github.com/w3c/csswg-drafts/issues/554
* https://github.com/w3c/csswg-drafts/issues/1973
* https://github.com/w3c/csswg-drafts/issues/1974
* https://github.com/w3c/csswg-drafts/issues/1975

There's been a hiatus working on these from the TTWG perspective because we've been working hard on getting TTML2 to CR, as well as backporting some fixes into TTML1 and generating a TTML1 3rd Edition CR, and updating the IMSC 1.0.1 profile to include TTML2 features, which will be IMSC 1.1.

A key part of the discussion was how we handle styling features that are in IMSC extensions but not supported by CSS yet. The main driver of discussion was actually namespaces and backwards compatibility in IMSC, but there's a strong correlation between the sets of features that fell into those two categories.

The TTWG's decision was to omit some features from TTML2 and defer their inclusion to a future version of TTML not before a clear mapping to CSS properties can be used. This has reduced the level of incompatibility between TTML2 and CSS somewhat, without removing the need for the styling features, and allowing existing extensions defined in IMSC (or EBU-TT-D) to continue being used.

Additionally, thanks to feedback from i18n, some advanced ruby support defined in TTML2 but not n CSS was removed from TTML2, on the basis that the immediate requirement was too weak and that there were some edge cases lacking clarity about exactly what implementations should do.

TTWG intends to keep working with CSS WG on the styling features required for subtitles and captions, and in addition the Media and Entertainment IG is setting up a task force to look at a similar or overlapping topic, so there may be additional input from others.

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