This issue is likely to apply to all cursive script text.

Text in 'cursive' writing systems is joined by overlapping glyphs with the appropriate joining forms. If those glyphs are made opaque, the joins should not become apparent. For example, N'Ko should look like this:

The GAP

When opacity is applied to text, some browsers produce dark patches where the cursive glyphs overlap. Here is an example in the Adlam script.

And here is an N'Ko example.

Gecko and Blink browsers show the unwanted overlaps. WebKit works as expected for Arabic, Devanagari, Syriac & N'Ko, but not for Adlam (Fulah), Mongolian, or Hanifi Rohingya.

Tests

Interactive test, Applying opacity to cursive text doesn't produce dark patches where glyphs overlap

I18n test suite, Text opacity

Contains similar tests for Arabic and Mongolian text, with the same results, except that WebKit browsers don't display Mongolian as cursive text, so the test is moot in that case.

Action taken:

Issue, Add note for text opacity applied to cursive scripts Closed.

Browser bug reports:

Gecko • Blink • WebKit

Outcomes:

2020-07-27: css-color was updated to clarify that opacity should be applied to a whole element, not to individual letters, so that this shouldn't happen.

Priority:

Marking as advanced, since this is a decorative feature.