Module support in TTML2

Good morning/evening,

In preparation for our upcoming meeting, I would like to better understand
the obstacles to adding support for modules to TTML2 -- such support is
currently being discussed as an addition to TTML3 [1].

Couple of points:

- modules are already implicitly used by profiles of TTML1 and TTML2, e.g.
IMSC, which could benefit from the clarification

- adding modules do not affect conformance of existing implementations
since foreign elements/attributes are pruned. In other words, modules
merely define hooks into the TTML algorithms

- adding modules to TTML2 would automatically make them available to TTML3

- definition and adoption of modules will be simpler if adoption of TTML3
is not required, since TTML3 may result in changes of conformance and take
longer to complete

Looking forward to the discussion.

Best,

-- Pierre

[1] https://github.com/w3c/ttml3/pull/30