The number is eye-catching but it's measuring how non-conforming the source code is, as opposed to whether it results in the intended DOM. This is why HTML validators largely went out fashion since it's more practical to consider source "valid" if it renders correctly in the browser. The way the browser handles non-conforming source code is part of the spec [1].

[1] https://html.spec.whatwg.org/multipage/parsing.html#parse-er...