Question

Is there a library (or jquery extension) that provides automatic fallback to jquery when no css3 is available (as in internet explorer 9 for example) ?

Answer 1

I think modernizr would work for you.

Modernizr is a small JavaScript library that detects the availability of native implementations for next-generation web technologies, i.e. features that stem from the HTML5 and CSS3 specifications. Many of these features are already implemented in at least one major browser (most of them in two or more), and what Modernizr does is, very simply, tell you whether the current browser has this feature natively implemented or not.

This library uses something called Polyfills for cases where an HTML5 or CSS3 feature is not supported.

polyfill (n): a JavaScript shim that replicates the standard API for older browsers

And good news for you, there is a polyfill for nearly every HTML5 feature that Modernizr detects. Yup. So in most cases, you can use a HTML5 or CSS3 feature and be able to replicate it in non-supporting browsers. Yes, not only can you use HTML5 today, but you can use it in the past, too!