https://drafts.csswg.org/css-color-4/#transparency

In some scripts, the letters are connected at the base, and fonts achieve joining by overlapping letters. Making each letter transparent can expose these overlapping joinings, which should be avoided. It would be useful to add some guidance on alpha compositing for cursive scripts to the Transparency section for implementations.

For more background information:

• alreq § 3.4.7.2 Transparency
• i18n tests on text opacity