def _ensure_html_header(response):
    # type: (Response) -> None
    """Check the Content-Type header to ensure the response contains HTML.

    Raises `_NotHTML` if the content type is not text/html.
    """
    content_type = response.headers.get("Content-Type", "")
    if not content_type.lower().startswith("text/html"):
        raise _NotHTML(content_type, response.request.method)
Check the Content-Type header to ensure the response contains HTML.

    Raises `_NotHTML` if the content type is not text/html.