The XHTML 2.0 draft [1] specifies the nextFocus and prevFocus attributes.  
 
In general i think that the forward tour should be the same route as the
backward tour, i.e. there are pairs of nextFocus and prevFocus links that
form a double-linked list. Anything else would cause a usability problem.
 
If you are assuming that the forward tour uses the same route as the
backward tour, than why do we need the prevFocus element?  The user agent
could easily build the backward path from the nextFocus attributes.  Also,
this would make it easier for the author.
 
Best regards,
Gottfried Zimmermann
 
References:
 
[1]
http://www.w3.org/TR/2004/WD-xhtml2-20040722/mod-hyperAttributes.html#s_hype 
rAttributesmodule