def _html(tag):
    """
    Prefixes the given ``tag`` with the XHTML prefix. Block files saved with newer versions of App Inventor will have
    this XML namespace prepended when read with the ETree framework.
    :param tag: An XML element tag
    :type tag: basestring
    :return: A new string prefixed with the XHTML prefix.
    :rtype: str or unicode
    """
    return '{http://www.w3.org/1999/xhtml}' + tag
