I noticed some HTML5 apps prompt IE 6-8 users to download Google Chrome Frame to use it. I always wondered why they didn't choose to gracefully degrade. Is it because some HTML5 are simply impossible to reproduce (via external libraries) in older browsers? Or does every new feature have a fallback that these apps simply didn't include?
There are many new features of HTML5 that just won't work on older browsers. You can attempt to mimic the functionality using JavaScript, and the Modernizr script library helps HTML5 applications degrade gracefully when they're run on older browsers. At the end of the day, however, it's like asking for a black and white TV to show 3D high-def movies.
The Google Chrome Frame is a way to basically run Chrome within IE so newer pages will display properly. This isn't a bad option, but it's not realistic to expect all users to install it.