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.
If you don't already know, the hgroup element is obsolete in HTML5.
Advice is now provided in the HTML spec on how to mark up subheadings, subtitles, alternative titles and taglines using existing and implemented HTML features.