Looks decent. The approach taken is the right one: we do something very similar with the Squire rich text editor (https://github.com/neilj/Squire) which I wrote for FastMail's webmail. Basically the browser can't be trusted to do any formatting itself, which is a slightly depressing state of affairs, especially as there has been zero improvements in this area for the last 5 years. I guess it's not shiny enough for browser devs to focus on.

Looking through the code, a few things jumped out that should be looked at:

* Native TreeWalker implementations are buggy in some browsers. In the end we decided it was safer to just implement the bit we needed ourselves (see comment at top of https://github.com/neilj/Squire/blob/master/source/TreeWalke...).

* The HTML sanitisation using document.implementation doesn't account for DOM clobbering so is currently bypassable with the right malicious content. I recommend using https://github.com/cure53/DOMPurify for this rather than writing your own. It's tricky to get all the edge cases right, so better to use something that's been reviewed by several people (and in DOMPurify's case also undergone a formal security review). Again, we use this at FastMail as part of our webmail.