We load a basic stylesheet to all users. This contains only normalisation and a few small enhancements to the user-agent stylesheet. To load the full experience only in modern browsers (grade-A and grade-X) we implement logic in the media attribute of the
<link>element that identifies the main stylesheet, loading the stylesheet only in browsers that recognise the properties of that media query. This technique is documented here: Cutting the Mustard with Media queries.
Graded browser support (cache)
Very clever technique for Progressive Enhancement that I discovered via this fantastic talk transcript (cache). Go read the whole piece, the Web deserves it.