[gethly]

FYI, IDEs and editors too don't use "textarea". Contenteditable essentially makes the web browser work like editor does.

[mg]

Contenteditable plus the CSS Custom Highlight API (which highlights ranges instead of elements) might indeed allow for a good solution. But I have not yet seen an editor that does that.

[hyperhello]

I built a prototype that does work. https://hypervariety.com/ScriptHighlighter/