but... the program ends up entirely compiled and executed as TTF bytecode in the end right, since it's entirely contained in the TTF font ? And TTF bytecode is only the following instructions : https://developer.apple.com/fonts/TrueType-Reference-Manual/... and I don't see anything related to SIMD in there

No that is not what is happening, HarfBuzz have a experimental Wasm shaper, so the font embeds some wasm code that tells Harfbuzz what to output.

https://github.com/harfbuzz/harfbuzz/blob/main/docs/wasm-sha...

oh ok, so it can't work in any system that doesn't use harfbuzz.. that's much less interesting than what I originally thought