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