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