> The rules could be expressed like this:
> 
> 1.	There can only be one h1, and it must be the first heading.
> 2.	The first heading after h1 must be h2.
> 3.	The first heading after h2 must be h2 or h3.
> 4.	The first heading after h3 must be h2 or h3 or h4.
> 5.	The first heading after h4 must be h2 or h3 or h4 or h5
> 6.	The first heading after h5 or h6 can be anything except h1.
> 

Assuming the context node is one of these headings:

following-sibling::*[h:is-heading(.)][1][h:level(.) gt 1 and h:level(.) le
h:level(current())+1]

where

h:is-heading() is {matches(local-name(), 'h[1-6]')}

and

h:level() is {xs:integer(substring(local-name(), 2))

(And add a condition on the parent element that there is an h1)

Michael Kay
http://www.saxonica.com/