def get_lang_tags(index_page):
    """
    Collect informations about language of the page from HTML and Dublin core
    tags and langdetect guesses.

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

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

    lang_tags = [
        get_html_lang_tags(dom),
        get_dc_lang_tags(dom),
        [detect_language(dom)],
        get_html_tag_lang_params(dom),
    ]

    return list(sorted(set(
        SourceString(normalize(lang), source=lang.source)
        for lang in sum(lang_tags, [])
    )))