IMSC is not a strict superset of SDP-US

[skynavga]

SDP-US Section 4.2 [1] states:

"R0008 - A document must contain a ttp:profile element where the use attribute of that element is specified as http://www.w3.org/ns/ttml/profile/sdp-us."

however, IMSC Section 6.10 [2] states:

"#profile permitted ... The ttp:profile element shall not be present."

This means that the statement made in the IMSC Abstract and Scope sections regarding SDP-US is incorrect:

"The text-only profile is a strict superset of [ttml10-sdp-us]."

[1] http://www.w3.org/TR/ttml10-sdp-us/#Use_of_Constrained_TTML_Feature_content_profile
[2] https://rawgit.com/w3c/imsc1/master/spec/ttml-ww-profiles.html#common-features

[palemieux]

The ttp:profile element was allowed all the way to, and including, CR2 -- possibly in part to support SDP-US. It looks like the prohibition of the ttp:profile element is a bug inadvertently introduced in an effort to simplify profile signaling. PR #129 proposes to revert the change and allow the ttp:profile element to allow SDP-US documents to be processed by IMSC1 processors.