As you design your site, keep in mind that different people will view your site on different types of computers, with different monitors, and at different resolutions.
Consider whether any of your users may still be surfing on a five-year-old computer with 256 colors and a 640X480 display (yes, there are still plenty of them!). Consider what happens to your site if someone changes the basic settings on their browser.
Try setting your computer display to 640X480 pixels and 256 colors. Try setting the default font to a very large size (older people often do that). Try setting your browser to the "always use my colors, overriding document" setting. Try setting your browser to not automatically load images. Does your design still work under these conditions? It may not be pretty, but is it functional?