> The specification would dictate the exact visual appearance,
> in a manner that can be overridden in CSS only. Although this would 

Unfortunately there is a very strong lobby on both the CSS (relevant)
and SVG (not directly relevant) lists to have repeatable appearances
because browser developers find that authors and users are completely
incapable of understanding the concept of implementation choices and
therefore market forces force them to make their presentation alike.
They therefore argue that if the specification doesn't mandate, they
would have to reverse engineer the market leader, making that the
specification, but not so well written a one.

I do agree that this is pandering to presentational markup and XHTML 2
really ought not to follow that line, even though CSS and SVG are 
definitely following it at the moment.