    THIS FAILS IN libxml2 2.6.29 AND 2.6.30 !!

    >>> from lxml.html import fromstring, tostring
    >>> from lxml.html.clean import clean, clean_html, Cleaner
    >>> from lxml.html import usedoctest

    >>> def tostring(el):  # work-around for Py3 'bytes' type
    ...     from lxml.html import tostring
    ...     s = tostring(el)
    ...     if not isinstance(s, str):
    ...         s = s.decode('UTF-8')
    ...     return s

    >>> doc_embed = '''
... ... ...
...
...
''' >>> print(tostring(fromstring(doc_embed)))
>>> print(Cleaner().clean_html(doc_embed))
>>> print(Cleaner(host_whitelist=['www.youtube.com']).clean_html(doc_embed))
>>> print(Cleaner(host_whitelist=['www.youtube.com'], whitelist_tags=None).clean_html(doc_embed))