<hgroup></hgroup> element was first introduced in HTML5 and Gecko 1.9.3. It represents the heading of a section. It defines a single title participating in the outline of the document as the heading of the implicit or explicit section that it belongs too.
Its text for the outline algorithm is the text of the first heading element of highest rank (i.e. the first
<h6> with the smallest number among its descendants) and the rank is the rank of this very same heading element.
Therefore this element groups several headings and hides them, except the main one, to the outline of the document. It allows the association of secondary titles, like subheadings, alternative titles or even taglines, to the main heading, without polluting the outline of the document.