Sanitize a string in order to avoid possible XSS using
        ``html5lib``.

def sanitize(self, content):
        """Sanitize a string in order to avoid possible XSS using
        ``html5lib``."""
        import html5lib
        from html5lib import sanitizer
        p = html5lib.HTMLParser(tokenizer=sanitizer.HTMLSanitizer)
        dom_tree = p.parseFragment(content)
        return dom_tree.text