def get_html_tag_lang_params(index_page):
    """
    Parse lang and xml:lang parameters in the ``<html>`` tag.

    See
      https://www.w3.org/International/questions/qa-html-language-declarations
    for details.

    Args:
        index_page (str): HTML content of the page you wisht to analyze.

    Returns:
        list: List of :class:`.SourceString` objects.
    """
    dom = dhtmlparser.parseString(index_page)

    html_tag = dom.find("html")

    if not html_tag:
        return []

    html_tag = html_tag[0]

    # parse parameters
    lang = html_tag.params.get("lang")
    xml_lang = html_tag.params.get("xml:lang")

    if lang and lang == xml_lang:
        return [SourceString(lang, source="<html> tag")]

    out = []

    if lang:
        out.append(SourceString(lang, source="<html lang=..>"))

    if xml_lang:
        out.append(SourceString(xml_lang, source="<html xml:lang=..>"))

    return out