Question

What are the valid html elements, if any, that can be contained within a <a> tag?

Answer 1

Inline elements ( a, span, strong, em among others ) can contain other inline elements and text nodes. An anchor can contain a span, which can contain a text node.

Generally, block-level elements may
contain inline elements and other
block-level elements. Generally,
inline elements may contain only data
and other inline elements. Inherent in
this structural distinction is the
idea that block elements create
“larger” structures than inline
elements.

From http://www.w3.org/TR/html401/struct/global.html

As noted in other answers, you can’t nest an a in an a.