The user could then press left and right to navigate among sibling HTML elements, while down and up traverse down and up the hierarchy. So for the tree 1(A(a,b,c), B(d,e,f), C(g,h,i)) to navigate to element 'e' from element 1  you would press DOWN RIGHT DOWN RIGHT.

If you can think of a more usable way to navigate a tree w/ the arrow keys, I'm all ears!