Here’s a few font experiments, more or less sensible, that you should be aware of:
Heikki Lotvonen’s text highlighting that is built into the font itself. Does not use HTML, CSS or Javascript.
llama.ttf that has an “AI” (LLM) built into the font itself, so when you write !!!!!!!!!!!!!!!!!, every ! is replaced with a word. (check out the links at the bottom of that page too)