First of all, the application performs CSS3 animations, text shadows etc.... so for older browsers another css file should be showed instead. I could do this with PHP server detection, but I have read about modernizr to perform browser detection on the client-side.
I would use graceful fallbacks so that user will get an approximation of the functionality if they can't view the full deal. If a browser cannot support a CSS declaration it won't try so it doesn't hurt to have those in the file sent to a browser that can't handle the animation.
That's what conditional tags and media queries are for.. e.g;
<!--[if IE 8]><html class="lt-ie9" lang="en"><![endif]-->
Relying on too many libraries though isn't cool.. I don't like looking at my cpu monitor and seeing 30% (from 5%) usage on a overclocked i7 on a website. ipad's won't like it either - especially the newer ones, theyre notoriously slow for "bleeding edge" html5/js/css.. I've been to websites on my old e7300 computer that would nearly max the cpu.. that's pathetic
only use jquery for things that actually save u time.. $('html').hasClass('lt-ie9'); might save u 1 second in typing but adds a lot more cpu cycle - when you start running more complicated apps and checks with numerous loops and what not it starts adding up fast. I recently made a puzzle with jquery and by that change (looping through 25 input boxes to check values) it would actually lag on slower computers