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

Thanks for beginning this process. I Chair this activity in TTWG. We do intend to provide a high level summary of the differences between TTML1 and TTML2 to aid the review process. You're looking at the current ED, which is fine to get started with; we plan to publish a new WD in November which the review request is targeted at.

The profile mechanism in TTML1 is working okay and has adoption to a greater or lesser extent; there is certainly a benefit in indicating which features a processor needs to process any given document, and potentially to choose which processor to use given a document. Usually this is not done in detail in-band at a granular level in the document but by reference to an external profile definition document or other specification such as [IMSC 1](https://www.w3.org/TR/ttml-imsc1/). The additional functionality in TTML2 addresses specific points raised by external groups wanting to adopt specific profiles of TTML, such as the difference between the signalling of document instance conformance to one or more profiles vs processor capability to process documents from one or more profiles.

The styling attribute vocabulary is in general based on CSS; in some cases there is styling functionality absent from CSS where TTML2 fills the gap, for example some of the detailed Ruby and text emphasis layout semantics, and tighter control of the sizing of blocks. We do have a [Charter](https://www.w3.org/2016/05/timed-text-charter.html) requirement to coordinate with the CSS WG and will seek review there also.

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