Afaik theres some language hints nowadays but its kinda hack
What happens if you want both single-storey "a" and double-storey "a" in the same document? You use a different font.
I won't even touch the fact that what you're talking about is just a stylistic difference, rather than a language based one, and will instead say this: What if you want the cyrillic letter А and the latin letter A, which are not just the same glyph, but literally visually identical looking in the same document? Oh wait both of those have separate UTF-8 codepoints. But if you want chinese and japanese characters which do not look identical in the same document, you have to resort to changing fonts? What if you're using an encoding that doesn't support specifying fonts? Your non-response doesn't solve anything and helps no one
Some fonts allow for both alternatives in them
Why is the language tag not used to signal a variant?
That doesn't help in a mixed Chinese-Japanese document
Why not? You don't have a single tag limit per document and can tag every mixed part with the appropriate language
That's not the only granularity of mixed text. A Chinese textbook about the Japanese language will have sentences where the languages are mixed
They should add separate code points for each variant and at least make it possible to avoid the problem in new documents. I've heard the arguments against this before, but the longer you wait, the worse the problem gets.