Most people never screw with the default settings of their computers. Fonts are typically displayed as black text on a white background.
Now, well, I hate that. I like white on black. It's easier to read, and even looks cooler (reminds me a little of the old green-screen computers.
So, I'm happily browsing around, and find an eBay auction that some poker player put up to sell ad-space on his face. Search there for "Tattoo my face" if you're interested, as I won't bother with a transient link like that.
And there was no text anywhere.
I decided that there must be text, so I selected it all and up popped the typical auction info. It just happened to be invisible.
Checking the source for the page, I found that in their styles they always specified background colors, and never a foreground color.
Well, that's pretty damn important. If you're going to force your users to stare at a white screen while on your site, at least have the common sense to tell them what foreground color to use for text. My computer doesn't know that you're changing the background on it.