HTML5Widgets is a JavaScript polyfill that implements a subset of the HTML5
Forms module in all browsers.  The script will only add support for the
different parts of the module when there doesn't exist a native
implementation - for example, at the time of this writing, it will implement
the datetime calendar widget in all browser except Opera, which supports it
natively.

More information on how this works is available at:

http://www.useragentman.com/blog/2010/07/27/cross-browser-html5-forms-using-modernizr-webforms2-and-html5widgets/