What happens if the DTD doesn't support them, or it the DTD
allows arbitrarily large and complex content for them? They
could end up taking the entire real estate available, especially
if the window is resized to 200x200.... or on a TTY. There will
always be cases where this proposal will fail. The main implemntation
problem is finding these cases, and dealing with them "correctly":
especially if there is arbitrary content. An embedded scrolling table
view widget is something I can understand, and see a need for, but I
don't think it will be possible to make header and footers "sticky" in
a way is always useful (or meaningful).
Hard coding anything to do with presentation is probably not
a good idea.