That's a good concern, but I think we can happily ignore it.

The DOM represents the tree structure derived from the markup by the browser, so dealing with malformed markup wouldn't be the problem of this extension, it'd be the problem of the browser it's integrating in to.

I'm finding malformed markup has not been a concern of mine for many years. It used to be bad, real bad, but now I cannot think of the last time that I had to deal with malformed markup while scraping.