This is actually two separate questions (input and output), and the answer to both is "yes".

On input (from browser to CGI) side, as mentioned above, various browsers act differently in how they format a POST. Using CGI.pm, I haven't had problems, though I've heard that Opera does some strange things with POSTS.

On the output (from CGI to browser) side, the CGI can emit whatever HTML/DHTML/XHTML/etc. it wants, and it's entirely up to the browser to do something sensible. In practice, it's a mess. Every release of every browser so far is buggy or incomplete when it comes to CSS (cascading style sheet) support, for example. Perlmonks isn't the right place to go for treatises on browser misbehavior. Try starting with A List Apart and following the links.