If you have a title with subtitle(s) or tag lines (i.e., more than one consecutive <h1>–<h6>), group them in an <hgroup>.
If you have a title with subtitle(s) and other metadata associated with the section or article, place both the <hgroup> and the metadata within a single <header> element.
When grouping headings in an <hgroup> element, the outline algorithm will mask the lowest level all but the highest level heading in the group from the resulting document outline.
It can only contain a group of <h1>–<h6> element(s), and it should be used for subtitles, alternative titles, and tag lines.