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"One or more ASCII digits." The second set of ascii digits should be marked as present only if the optional decimal point was present. Otherwise this could be read as requiring two or more ascii digits, i.e. width=6% would be invalid. Likewise with the other region attributes. It makes sense to abstract this into a 'webvtt percentage value' or something similar so it can be described once an referenced from all three attributes.
Ah, I see you already do this in the parsing section. I was worried about the description in section 4.4.1. Maybe these can just be 'an optional decimal'? I was mislead by the level of detail into thinking this was normative. :)
It is normative, but it doesn't apply to implementors. It applies to the author or WebVTT generator, as well as validators. This this is a bug. The spec says that 6% is invalid (although the parser might accept it). It should say: 4. Optionally: 1. A U+002E DOT character (.). 2. One or more ASCII digits.
Agreed, it's a bug. Thanks for noticing.
*** Bug 24069 has been marked as a duplicate of this bug. ***
Fixed in https://github.com/w3c/webvtt/commit/730581507769f5022adca472c8d26c71f623f4d6