Browser degradation is the most crucial problem we face when enjoying all the cool features in HTML5 and CSS3. Thanks to the work of some really talented and smart developers we are able to fix the holes in these browsers using polyfills.
What is a polyfill ?
polyfill: A shim that mimics a future API, providing fallback functionality to older browsers.
To start off here is a link to Paul Irish’s amazing polyfill resources list:
And here is Modernizer
These two links are the most comprehensive approachs to regressive enhancement I have found.
I also just heard that Modernizr will be shipped with ASP.NET MVC 3 Tools along with jQuery !