I might be wrong, but I'm pretty sure Google Docs is already using a completely custom implementation of word wrapping, text layout, text selection, cursor placement, etc. It's not like it's just a <textarea /> with some CSS styles. Likewise, they seem to already have completely separate DOM elements that are invisible to normal browsers but can be read by screen readers. Based on the DOM on the new read-only preview document they link to, it looks like they will continue to use traditional DOM elements for some of the editing UI (just not the actual WYSIWYG editing area) and for screen readers.