> The East_Asian_Width property is not intended for use by modern terminal emulators without appropriate tailoring on a case-by-case basis. […]
It’s about a very specific typographical custom in a specific set of languages. Terminal emulators have piggy-backed onto it because it’s fairly close to what they want and definitely better than nothing, but if they limit themselves to this definition, they do a bad job.
> you shouldn't need knowledge of the type-face to know the size of the printed character
To see the most extreme example, look through Google Fonts for U+FDFD <https://fonts.google.com/?preview.text=%EF%B7%BD> and see the radical differences in aspect ratio (and size!). In some other scripts there are multiple ways of writing things which have quite different width characteristics. In English we have different accepted and understood letterforms for a, g, v, z and probably others, but they’re all roughly the same width. But this isn’t always true in other scripts. My name is written in Telugu as క్రిస్ (kri s), but that r curve can be drawn below or to the left (both seen among <https://fonts.google.com/?preview.text=%E0%B0%95%E0%B1%8D%E0...>), and even sometimes a little to the right; and you don’t actually have to write it with a conjunct, క్ రిస్ (k ri s) means the same thing and there used to be some fonts or apps that wouldn’t render conjuncts. (Encodingwise, I inserted a ZERO WIDTH NON-JOINER after the {k, suppress vowel} and before the {r, -i} to simulate this effect. And then HN turned the ZWNJ into a space. Sigh. I should ask them once again to stop doing that.) Of these three different legitimate renderings of the same text, one works in two cells, one wants two and a half, one needs three.