HTML 5

HTML5 is not something that is supported or isn't supported.

It's not even a complete specification yet. Browsers may or may not support some of HTML5 features. Legacy browsers may have no support at all for newer features introduced with HTML5. As long as you don't use any of the new features introducted in HTML5, your document will be compatible with legacy browsers too.

If you plan to use some of the new features not supported by legacy browsers, and at the same time making them compatible with your site, you need to choose a different method to display your content.

If you want to make HTML5 compatible with most browsers, I suggest you to use some good tools, likeBoilerplate and Modernizr.

Also, I really suggest to read this explanation about HTML 5.

To get the most out of HTML 5, I have linked many good resources in this anwser.