Add ebutts:linePadding vocabulary to TTML2

[palemieux]

ebutts:linePadding as specified in EBU-TT-D and referenced by IMSC 1.0.1 has been adopted by the industry.

TTML2 provides similar capabilities by adding support for padding on span (see https://www.w3.org/TR/ttml2/#style-attribute-padding). This approach however has a number of drawbacks:

• it uses a syntax that is significantly different than that of ebutts:linePadding, complicating the life of users and implementers alike, without improved CSS support (since CSS does not support the proposed TTML2 syntax)
• it is significantly more complex to implement than ebutts:linePadding (since each span can have a different value) without having demonstrated why such complexity is needed for subtitles/captions (or timed text in general)

The ebutts:linePadding vocabulary and syntax should be added to TTML2.

[skynavga]

I disagree (with the proposal). TTML2 should not incorporate non-TTML namespace vocabulary. Further, there is no requirement that TTML2 be a syntactic superset of industry profiles.

[skynavga]

I believe we agreed at today's F2F that we would not add ebutts:linePadding to TTML2. Is this correct? If so, then could the reporter close this?