Most proposed features of HTML5 and CSS3 are not supported in browsers yet, but you can experiment with a few. There are style sheets and scripts written for older browsers that help them recognize some of the newer elements. For example, you can use the [html5shiv together with css [URL="http://necolas.github.com/normalize.css/"]normalize to help browsers cope with them. Or yo can use [URL="http://modernizr.com/"]Modernizr](http://code.google.com/p/html5shiv/).
Personally, though, all this seems like it's jumping the gun and abandoning the principles of graceful degradation / progressive enhancement, as you end up replying of JS/CSS for content to be available, which is a kind of heresy.