Return list of `keywords` parsed from HTML ``<meta>`` tags.

    Args:
        index_page (str): Content of the page as UTF-8 string

    Returns:
        list: List of :class:`.SourceString` objects.

def get_html_keywords(index_page):
    """
    Return list of `keywords` parsed from HTML ``<meta>`` tags.

    Args:
        index_page (str): Content of the page as UTF-8 string

    Returns:
        list: List of :class:`.SourceString` objects.
    """
    keyword_lists = (
        keyword_list.split(",")
        for keyword_list in parse_meta(index_page, "keywords", "HTML")
    )

    # create SourceStrings from the list of keywords
    return [
        SourceString(keyword.strip(), source="HTML")
        for keyword in sum(keyword_lists, [])  # flattern the list
    ]