Bengali অ্যা and এ্যা [æ] are selected as a single grapheme by ::first-letter.

[r12a]

Test link(s):

test

Results:

The test was run with the following pre-installed fonts:
Windows: Shonar Bangla, Arial Unicode MS, Nirmala UI, Vrinda
Mac: Bangla MN, Bangla Sangam MN, Kohinoor Bangla, Tiro Bangla, Baloo Da
Also tested with Noto Sans Bengali and Noto Serif Bengali on the Mac.

• Gecko: ✅❌ Mac: fails for Bangla MN, and Bangla Sangam MN, but passes for the other 3 fonts. Windows: works fine for all fonts. Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:95.0) Gecko/20100101 Firefox/95.0
• Blink: ❌ Mac & Windows: fails for all fonts. Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.93 Safari/537.36
• Webkit: ❌ Mac: fails for all fonts. It was not possible to apply the Noto fonts. Browser: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.1 Safari/605.1.15

Related tests: Styling initials