NOTE-css-potential: CSS1.0 cleanup: break

I think, from reading the draft, that since <BR> takes up no space, these
two would be equivalent:

BR { break: line-after; }
BR { break: line-before; }

I think it would also be useful if I could specify a break both before
and after an element.

I think that it would also be useful to be able to specify both
block-before and block-after; in fact, that needs to be implicity
when you see a block-level element nested in another block-level
element.  (At least, that seems like a useful recovery technique
for a browser to use; I'm not sure that validators should accept that.)