[CryZe]

You could compile an actual syntax highlighter to WebAssembly and embed that into the font. That would address all the issues. However, WebAssembly is an off-by-default experimental feature for harfbuzz, so at least for now the current solution is the only feasible one if you want it to work in most places.

[jfk13]

Another issue you'd run into, I suspect, is that in practice text engines/editors/etc will often repaint only fragments of the content, and so the syntax highlighter would not necessarily see all the context it needs.

E.g. if you have a comment that starts with "/*" and ends several lines later with "*/", but the editor decides to just redraw a line in the middle, the highlighter won't "know" to use the comment color as the text-drawing operation won't include the delimiters.

(This issue already exists in a small way, as mentioned in https://news.ycombinator.com/item?id=41251114, but it'd only get worse as the syntax highlighter becomes more sophisticated.)