indispensable tools for HTML5 and CSS3 - Modernizr and HTML5.js

This is a js tool that detect browser's HTML5 supported features.

http://modernizr.com

as example, once loaded js provides variables that can branch your code for HTML5 features supported by users's browser:

if (Modernizr.geolocation) {
code
}

The second tools is Remy Sharp's HTMLShiv3 is hosted on http://html5shiv.googlecode.com/svn/trunk/html5.js
it allows to recognize browsers before IE9