Start hierarchical level subsections within a section (D2). Each subsection:
Subsections may be nested to eight levels. D3 subsections must occur after a D2 element, and each subsection must occur within a successively higher level section. For example, if you use a D4, it must be preceded by at least one D3 and D2.
A HEAD element is required for each subsection, but text is optional in all subsections.