This issue is applicable to many Indian languages.

When upright characters appear in vertical lines of text, browsers need to tailor grapheme cluster rules so that full orthographic syllables represented as conjuncts, are kept together.

See requirements at: Indic Layout Requirements, but add to those the points made in Typographic character units in complex scripts.

The GAP

Gecko appears to segment on grapheme clusters. Blink, and Webkit keep conjuncts together. But all engines have problems with vowel-signs, especially but not only pre-base vowel-signs.

Priority

Marked as advanced, since vertical text is not a common use case. In writing sytems like modern Tamil, it is even less common, since conjuncts are rarely used.

Tests

Interactive test, When devanagari characters are rendered upright in vertical text, orthographic syllables containing conjuncts are kept together

Action taken

Gecko • Blink • Webkit

Outcomes

All three major desktop browsers now keep the conjuncts together, however the vowel signs attached to those orthographic syllables are not kept with the rest of the syllable.