I echo tracknut's comments, and would add that, in addition to the things that do not work across browsers, many of the things that do work in HTML5 and CSS3 require browser specific code, because they have not been implemented in a standard fashion. That will eventually be resolved, but right now it is clunky.
Also, bear in mind that IE8 and below (which still have global usage levels of 10%+) are not HTML5/CSS3 compatible anyway. So you need to ensure graceful fall back on non-compliant browsers.
Furthermore, the bits of HTML5/CSS3 that are well supported, like border-radius, can be used in HTML4/CSS2 documents anyway. You will get complaints from the validator, but it will work.