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http://dev.w3.org/html5/webvtt/#h3_conformance-for-authors [[ The Parsing section of this specification defines in some detail the required processing for valid and also for invalid documents. It is a little more tolerant to author errors than the syntax allows, so as to reject less documents and provide for extensibility. However, authors must not take advantage of it. Only documents that follow the Syntax specification are valid. ]] "However, authors must not take advantage of it." This requirement make no sense. I suggest it is dropped.
I agree, the syntax itself already restricts what aspects of the parsing authors can and cannot take advantage of.
Yes, we should say instead that the syntax is normative and documents must conform to the syntax. (We also need to change 'less documents' to 'fewer documents', ahem).
Closing this, since it has been fixed in https://github.com/w3c/webvtt/commit/0d63b769c6bc355a4d1a90a6eb0e8f416f5a22f7