To some degree; but not extensively. I heavily use it for interpreting HTML content in some private projects (using DOMParser.parseFromString from cha -r), so I have probably caught the most obvious errors by now. But it has no proper testing yet.
An integration of [html5lib-tests](https://github.com/html5lib/html5lib-tests) would be a good first step in this direction. Also, a fuzzer for decoderstream has been on my to-do list for a long time.
