Anything based on positioning within a tree is fragile, and in such
cases, it would be better to hook the style onto an attribute, or
through a more general name like "LI,B" (which references a class of
<B> elements). That does not mean they are not useful though.
I think you'll find that in the general case, people will not use such
things anyway. In EBT's DynaText, for example, it is *very* rare to see
people setting a style for one particular element, even though it is
possible.