There is a route you can go that doesn’t use contenteditable, except as a small box at the cursor position to handle IME; it’s what Google Docs does. At my most recent job (stealth start-up) we built an editor like this, painting our own selection and everything. It makes certain things easy or possible — custom text wrapping, tab stops, custom handling of cursor and selection with regard to embeds. It doesn’t get you very far on mobile web, though.