At this point I'd like to re-iterate an earlier point: it's almost certainly possible to polyfill the majority of HTML5 and CSS3 features for older browsers but the resulting user experience will be heavily laden with JavaScript and potentially less usable than it would be without the polyfills. Needless to say, it's important to consider the performance implications of such a choice. Just because you can, doesn't mean you should!
Furthermore, even without polyfills (which we shall look at shortly), in my experience, adding, testing, and configuring IE specific CSS code to make IE6 and IE7 (and to a lesser extent IE8 and IE9) render pages so they look as similar as possible to a modern standards compliant browser takes at least as much time as visually enhancing a site for modern browsers—just far less enjoyable! Is that how you or your client want to spend the allocated development time?