::first-letter will select grapheme clusters (ie. base + combining characters) as a basic unit, including prescript and double-sided vowel-signs.

[r12a]

test

Results:

• Gecko: ✅ Doesn’t highlight any initial beginning with a combining character. Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:94.0) Gecko/20100101 Firefox/94.0
• Blink: ✅ Highlights initial combining characters, too. _Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_157) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.69 Safari/537.36
• Webkit: ✅ Same as Blink. _Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_157) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.1 Safari/605.1.15

Related tests: Styling initials