Color blindness
Damnit. After all these years people still don’t realize basic ideas in web design: if you’re going to specify one color for my page, specify them all! Do you think it’s safe to assume anything about my default colors? No! I’m using Konqueror, the only “good” browser that runs on any flavor of Unix, and it has themes that set my default colors. The hi-contrast scheme I have sets link color to yellow. Now, if you set your background to white, and don’t set your link colors, I can’t see anything! Your text vanishes into near-nothingness. The worst offender: Yahoo!. You’d think the pioneers of the portal/search idea would know how to write HTML.