When letter-spacing is applied to Tamil most consonant clusters will be split, but not the conjuncts க்ஷ ஶ்ரீ and ஸ்ரீ

[r12a]

test

The number of conjuncts supported by a font may be different from another font, so set the font to the same value across all browsers.
The Tamil conjuncts are highlighted.
From the pulldown labelled "Run a test" choose "Letter-spacing".
Check the results.
Test passes if all clusters are separated, except for the highlighted conjuncts.

• Gecko: Chrome/89.0.4389.90 Mac OS X 11_2_3 ✓✘ Everything works as expected, except for the conjunct ஸ்ரீ (U+0BB8: TAMIL LETTER SA⸱U+0BB8 U+0BCD U+0BB0 U+0BC0: TAMIL LETTER SA, SIGN VIRAMA, LETTER RA, VOWEL SIGN II⸱U+0BCD: TAMIL SIGN VIRAMA​⸱U+0BB0: TAMIL LETTER RA⸱U+0BC0: TAMIL VOWEL SIGN II​).
• Blink: Chrome/89.0.4389.90 Mac OS X 11_2_3 ✓
• Webkit: Chrome/89.0.4389.90 Mac OS X 11_2_3 ✓