ISSUE-446: Should be a subset of IMSC1

The TVTT "profile" is a subset of IMSC 1 and therefore, if this is taken as a start, should simplify the mapping process for implementers considerably. With a TVTT document you just have to read the section 2.3 which explains the mapping from TVTT to WebVTT.

If the source document does not comply in certain areas with the TVTT document you just can read selectively parts of the pre-processing section 2.2.

It has been discussed between the authors of the document to restrict from the start the complete mapping to a certain profile (e.g. IMSC 1). But as a result of the discussion this approach has been found to be to constrained and not sufficient for current operational mapping requirements. There are TTML documents that do not comply yet with IMSC 1 but nevertheless needed to be mapped to WebVTT.

It should also be taken into account that the TVTT profile primarily fulfills a didactical purpose to make the complexity of the mapping process manageable and to “distribute” it over several steps. These steps may be used by an implemented mapping algorithm but dependent of context and scope imoplemented processing may be different (as long as the results are the same).

Of course the TVTT profile may also be taken as a start for authoring but this very much depends if the author is willing to take out features that are not supported by WebVTT.

It may be helpful to add a part to the document and note that the TVTT profile is already close to or subset of EBU-TT-D and IMSC 1 so it makes mapping considerably easier if they use these two profiles.