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

A new TTML2 WD has been published at https://www.w3.org/TR/2016/WD-ttml2-20161117/

Here is a brief descriptive overview of the changes between TTML1 and TTML2:

The TTML2 specification updates the TTML1 specification by adding vocabulary and semantics to address more of its core requirements. 

In summary it supports continuous animation of styling and positioning, dereferencing and user of embedded or externally located resources such as images, fonts, and audio, a more easily extensible metadata mechanism, the ability to specify alternate initial values for styling, a more useful profile mechanism, a conditional processing mechanism and a serialisation format for resolved temporal subsections (intermediate synchronic documents). The styling vocabulary has been increased to support advanced text layout requirements such as Ruby, text emphasis, images, backgrounds, shadows and in the next iteration we plan to support basic audio mixing by specifying gain and pan. 

At a functional level this means that TTML2 supports text layout requirements for every script globally, is more fit for purpose in terms of profiling and use by other downstream systems, supports audio (video) description workflows and allows for more predictable rendering by means of referencing specific fonts to use. Conditional processing, such as semantic or transformation processing, based on parameters, media queries, language selection etc are now also possible.

Several semantics defined non-normatively in TTML1 have been promoted to being normative. All TTML1 documents are valid TTML2 documents; however, TTML2 documents that specify @version 2, may not be valid TTML1 documents. Where there is feature overlap between deprecated and new features, such as tts:origin and tts:position, precedence rules are defined. Where appropriate references have been updated, including normative references.

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