This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
The book contain information around it :)
You will want to leverage Modernizr and do something like:
The main problem is what do you do in these older browsers...
and for that, you need to see what will be the right solution for your case so your site/web app will do 'graceful degradation' and the user won't suffer.