Understanding Progressive Enhancement
Start with your content peanut, marked up in rich, semantic (X)HTML. Coat that content with a layer of rich, creamy CSS. Finally, add JavaScript as the hard candy shell to make a wonderfully tasty treat (and keep it from melting in your hands).
Aaron Gustafson has a great write-up over at A List Apart about developing websites using progressive enhancement and why content is more important then degradable browser support.