Mozilla/Firefox and Opera are good as far as support goes. Internet Explorer still has loseriffic CSS support, but supports proprietary tags that aren't even clearly marked as such OR established CSS tags. For shaaaaame.
CSS is an excellent tool when used properly. See
The CSS Zen Garden for a good example.
Using linked CSS files allows you to separate your style and content information, makes your files smaller, and make sweeping style changes comparitively easily.
I prefer XHTML 1.0 to HTML 4.0 anyhow.