no, I’m saying exactly the opposite. Not for markup, only for content. That’s what I mean by “lexically”. Take a look at the spec and you’ll see the places where Unicode is supported is in content segments only, not markup or whitespace used to delimit markup.
The PR that @jgm links to does exactly what you suggest: “specify particular whitespace characters as needed” in the lexical/markup contexts instead of the ASCII “whitespace character” set.