You demo the result in GIMP (with a Wasm runtime linked(?)) and in the font viewer FontGoogles, and there's someone else who is playing around with it in gedit. There's no way to make use of this in, say, Firefox, yet, even though it ships with Harfbuzz, right?
I could not get it to work when I looked into it last, for Gimp I actually installed it globally, I think neither Chrome or Firefox uses the system Harfbuzz. Going by the dependants of Harfbuzz on Arch [0] it might be possible that it works in Chromium with a custom Harfbuzz, but it is not something I have tried. And since it is still experimental it is not built by default in any distribution I know of.
This is fantastic! These days, we have learned that no product can be taken seriously if it doesn't use LLMs somehow, so also based on Harfbuzz-Wasm, here's a font which is also an LLM: https://news.ycombinator.com/item?id=40766791
I did it with the Harfbuzz shaper which now have experimental support for embedding WebAssembly programs to shape fonts.
Talk where I show it off: https://www.youtube.com/watch?v=Ms1Drb9Vw9M
Source code: https://github.com/Erk-/programmable-fonts
You can also see actual uses of this WebAssembly embedding to show that is not just for fun here: https://github.com/harfbuzz/harfbuzz-wasm-examples