What happens if CSS isn't supported?

It's best to make sure the site "degrades gracefully" and looks okay with no style sheets. You can turn off style sheets in your browser preferences, so always make sure to preview your pages with and without style sheets to make sure that they're viewable both ways. For most things, it isn't a problem. Perhaps with CSS text might be 16pt lime-green with courier font and without it defaults back to normal black text. That's not such a big deal and the page remains usable both ways. Of course if you're using style sheets for element positioning, it gets a lot harder. For that I normally use an underlying TABLE so that if style sheets are off the table controls the page layout. Then each table row or cell is set as an individual DIV so that things can be more precisely controlled with absolute positioning.

I agree with Maverick. Just check your site with CSS turned off and if it looks decent, don't worry about those few visitors who don't have CSS enabled. You can't meet everyone's requirements without seriously limiting your design options. If only a few people don't have CSS enabled, don't worry about them, IMO.