Consonant clusters with a visible virama should only have the first consonant+virama highlighted

[r12a]

This issue is applicable to most languages that form conjuncts from consonant clusters using a visible virama.

When the start of a line contains a 2-consonant cluster that uses a visible virama, ::first-letter should highlight only the first consonant+virama. This corresponds to a grapheme cluster, as defined by Unicode.

Specs:
css-text-3 CSS uses the concept of 'typographic character unit', which defaults to a grapheme cluster. The spec doesn't provide details about the support needed for each language.

The Unicode Consortium defines grapheme clusters to, by default, stop after the first virama in a cluster.

Tests & results:
Interactive test, When ::first-letter is applied to Devanagari the browser will NOT select a 2-consonant cluster as a unit if the virama is visible

Interactive test, When ::first-letter is applied to Bengali the browser will only select the first consonant+virama in a cluster if the virama is visible

Gecko only highlights the initial character+virama. Blink, and Webkit select all clusters as a single unit, whether or not they are conjuncts or are rendered with a visible virama.

Browser bug reports:
Blink • WebKit

Priority:
This choice needs to be discussed. If a cluster is rendered as a conjunct, it should be selected as a single unit. If, instead, the virama is displayed (ie. it is not a conjunct) then only the initial part of the cluster should be selected. Since the underlying code points are identical if a cluster is displayed as a conjunct or instead with visible viramas, it's not possible to distinguish one situation from another by working with the code points. In the absence of a technical solution that examines what the font used does when rendering, this is a difficult problem to solve. This priority rating says that, in the absence of a perfect solution, it is better to select a whole non-conjunct cluster than to break a conjunct. So it is labelled advanced, rather than basic.

[r12a]

The first comment in this issue contains text that will automatically appear in one or more gap-analysis documents as a subsection with the same title as this issue. Any edits made to that comment will be immediately available in the document. Proposals for changes or discussion of the content can be made in comments below this point.

Relevant gap analysis documents include:
Bengali • Devanagari