Another thing in the category of abusing font capabilities is Harfbuzz (OpenType engine) WASM, which if compiled into an application allows fonts to basically run any code. A few examples are an LLM or a Tetris font. Apparently this WASM support is in the font rendering for Chrome and Firefox supposedly.

I was really tempted to try to use it to make a Harfbuzz OS.

https://fuglede.github.io/llama.ttf/