In bad_url is:

    def bad_url(url_status: int) -> bool:
        if url_status == -1:
            return True
        elif url_status == 401 or url_status == 403:
            return False
        elif url_status in (503, 504):
            # Usually transient
            return False
        elif url_status == 999:
            # Returned by some websites (e.g. LinkedIn) that think you're crawling them.
            return False
        elif url_status >= 400:
            return True
        return False

There's 3 errors here that are errors that have been hardcoded to not be errors. I noticed this because we have URLs that don't work. That always get a 504 but htmlproofer doesn't pick up. Given recent changes where you can now ignore all of a given code using raise_error_excludes: and *, I think that these 503, 504, and 999's should be removed so they can be configured by the user.

If you agree with the change @manuzhang, I'm happy to do it. If not then I think that the raise_error_excludes docs should be changed so they dont contain:

      raise_error_excludes:
        504: ['https://www.mkdocs.org/']

as 504 is always ignored at the moment and that bit of configuration while valid is also misleading.