The Unicode concept of 'grapheme cluster' currently fails to represent the small number of conjuncts that are used in modern Tamil, ie. kṣa க்ஷ and the two alternative sequences for srī, ஶ்ரீ and ஸ்ரீ. This means that various editing operations, line breaking algorithms, vertical text, etc. are liable to break text at the wrong point when those conjuncts are used. For more details, see the relevant sections.

Indic Layout Requirements provides a grammar for indian orthographic syllable boundaries which works for the consonant clusters in Tamil which don't use conjuncts.

Specs:
CSS uses the concept of 'typographic character unit', rather than grapheme cluster, in its specs with the explanation that these cases are beyond the scope of the grapheme cluster concept and that implementations should provide appropriate support.