TTML2 backwards compatibility with TTML1

I thought we had a (perhaps implicit and unspoken) requirement that TTML2 would be backwards compatible with TTML1.  That is, all conformant TTML1 instance documents would also be conformant TTML2 instance documents.

However, we are making decisions that are at odds with the above, e.g. this week's change to constrain origin and extent in issue 239 and its PR: https://github.com/w3c/imsc/pull/240  There are other examples.

I think we need to all be on the same page about the above requirement.

My personal view is that unless something is really broken and unusable/untestable from TTML1, that this requirement should hold.

Perhaps we can discuss this tomorrow before committing any more changes that are at odds with it?

                Mike

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Michael A DOLAN
TBT, Inc;  PO Box 190
Del Mar, CA 92014
+1-858-882-7497