It is unclear to me why the behavior defining what happens when a timing property is absent is in a NOTE. Shouldn't this be normative? Or if we rely on the underlying normative behavior of the TTML attributes, the note should say it.
That's the reason, because it is already defined in TTML. Happy for the note to include text that says that it is paraphrasing the TTML-defined semantics.
It is unclear to me why the behavior defining what happens when a timing property is absent is in a NOTE. Shouldn't this be normative? Or if we rely on the underlying normative behavior of the TTML attributes, the note should say it.