Customisable counters are needed for many languages.

There is no general support for counters that use the Tamil script or digits. There is also no way for users to create their own counter styles, or to modify things such as prefix/suffix assignments for particular contexts.

FIXED !

This gap is now fixed. For more details, see this GitHub issue, which is being used to track this gap. Please add any discussion there, and not to this issue.

What follows are Tamil-specific notes.

For Tamil the numeric style is most common, although there is also a question about whether an alphabetic counter style needs to be defined for Tamil. An additive style is used in older texts.

Specs:
predefined-counter-styles Two Tamil counter styles are defined in this document: tamil (numeric) and ancient-tamil (additive).

css-counter-styles-3 All of these counter styles could be defined by users if the Counter Styles spec's generic mechanism for defining counter styles was implemented. The spec is essentially done. The numeric style is defined explicitly in this specification; the additive style, however, relies on the user-defined mechanism in order to be applied.

Issue, Are alphabetic counter styles common in Tamil content? Open.

Tests & results:
I18n test suite, Simple numeric.
The numeric counter style is only supported by Gecko. Blink and Webkit do not support it.

I18n test suite, Ancient-Tamil.
Neither Gecko, Blink, nor Webkit support it.

I18n test suite, Basic custom counter styles.
The spec is essentially done, but so far only Gecko has implemented it. Blink and Webkit do not support it (however, we are hopefully close seeing this feature supported in Blink, too).

Browser bug reports:
Blink

Priority:
The impact of the lack of support cited here is mediated by the tendency to use western counters, however there appears to be a desire for native styles (at least the numeric) to be made available. So the impact is being marked as advanced.