[Cthulhu_]

The real question is whether you want to invest the time needed to create a fallback for those users. Websites that use the new CSS things as a primary thing (like this presentation) won't work, but sites where the newfangled CSS things are used just as embellishment - dropdowns, sliding menus, spinners, you name it - will remain functional, just not as pretty.

tl;dr graceful degradation / enhancement.