Changes to better support accessibility via ARIA

These changes should close #304
The problem is subtle.  a dfn element encloses a term.  the *defiinition* is not what is inside the dfn.  it is the surrounding prose if it is inline, or it is the next dd element if the dfn is in a dt.  A user using assistive technology can find it challenging to follow the chain from a term that is elsewhere in the document to the source of the term (the dfn) and then on to the actual definition.  These changes are designed to make that easier.