Add note about compatibility with DAPT #622
Description
Some observers may be puzzled about whether DAPT documents are conformant IMSC documents, or vice versa. It would be helpful to add:
- In §8.14.1 Profile Signalling - General: that DAPT documents would include the DAPT profile designator in addition to the IMSC profile designator
- In §I Compatibility with other TTML-based specifications: a new section on DAPT
Broadly, it is possible for a DAPT document also to be an IMSC document. The DAPT #serialization requirements are compatible with the IMSC §8.4 Document Encoding "SHOULD" statements. From an IMSC perspective, DAPT metadata is treated as per §8.5 Foreign Element and Attributes as within the set "elements and attributes that are neither specifically permitted nor forbidden by a profile". From a DAPT perspective, any IMSC-permitted vocabulary is also permitted.
A mirror section in DAPT would be helpful too.