https://drafts.csswg.org/css-text-decor-3/#text-underline-position-property

Here are the results of applying text-underline-position: under to some random text with a bunch of arabic script fonts i have on my system in Canary (today). I also applied text-decoration-skip: ink just because it shows collisions more clearly (it didn't affect the results). All text is set to 30px font-size.

The result is font-dependent. Some don't clear the bottom of the text. (And generally the line seems a little thick to me.) I didn't find any difference whether the language was set to en, ar, fa, or ur.

The following shows that for vowelled text the result is unfortunately much worse.