Turning TTML2 into Hobo Stew

As I predicted, the initial request to incorporate itts:fillLineGap into
TTML2 (#429 <https://github.com/w3c/ttml2/issues/429>) has now transformed
into a request to incorporate the vocabulary of every profile that extends
TTML1 or IMSC1 into TTML2 based solely on the argument that "the industry
does it".

I find these proposals extremely troubling, and in direct opposition to
longstanding design decisions about the nature of TTML2.

Let me make clear one of those design decisions: that TTML2 will be
syntactically backward compatible with TTML1 AND will define new extensions
to TTML1 in existing TTML namespaces (and not non-TTML namespaces).

TTML namespaces do not include IMSC namespaces, do not include EBU-TT
namespaces, do not include SMPTE namespaces, and do not include any other
random namespace that someone happens to claim is used by "the industry".

If I was willing to consider adding a single attribute in the itts
namespace previously, I am categorically opposed to adding attributes from
other namespaces as well, which means, at this point, that I am
categorically opposed to adding any IMSC namespace. So I withdraw my prior
possible consideration of adding itts:fillLineGap, and now stand opposed to
that original proposal.

If industry defined profiles that extend TTML1 want to use TTML2, then they
need to map their extension vocabulary to TTML2 defined vocabulary,
changing the namespaces and names of that vocabulary as required.