@classmethod
def sanitize(cls, url):
    """Clean and validate urls that should be verified as broken links."""
    if not url:
        return ''
    if url.startswith('#'):
        return ''
    if url.startswith('mailto:'):
        return ''
    if url.endswith('#respond'):
        return ''
    if cls.must_ignore(url):
        return ''
    if url.startswith('//'):
        url = cls.base_url_protocol + ':' + url
    elif url.startswith('/'):
        url = cls.base_url_protocol + '://' + cls.base_url_domain + url
    url = url.strip()
    return url
