Require HTML5 to use a website

Question

Theoretically, could you detect if a browser supports HTML5 and prompt the visitor to upgrade to IE9/Firefox/Chrome/Safari/Opera if it doesn't support some features like CSS3 or audio/video tags?

I'm developing a hobby music site in a mostly walled-garden and would like to prevent people with anitquated browsers from accessing/logging into the site.

Mass appeal is not a requirement as the site is for a niche and maximizes JavaScript and HTML5 use.

Answer 1

I'd suggest using the modernizr javascript api to detect the HTML5 features you desire.

http://www.modernizr.com/