Extensions should be easier to identify

[cconcolato]

In general, it is hard to identify the differences between IMSC1 and IMSCvNext.

Section F defining extension fragment identifiers is a copy of the one in IMSC1. It should either indicate that:

• the extensions designators of IMSC1 are used (full URL)
• it defines the same identifiers but with the IMSCvNext base URL
• it reuses the designators defined in TTML2
• or the section should be removed

Similarly, the first 4 subsections of Section 7 are copy/paste of IMSC1.0.1. I understand the need to have a self-contained document (although that's not the case regarding TTML2). I would suggest adding a sentence in each subsection indicating whether the text for each extension is identical to the one in IMSC1 or not.

[palemieux]

@cconcolato The provisions are intended to be substantively identical unless noted otherwise in Annex L.

[palemieux]

Section F indicates that:

The following sections define extension designations, expressed as relative URIs (fragment identifiers) relative to the IMSC 1.0 Extension Namespace base URI.

Propose to add note that this is specifically not IMSCvNEXT namespace.