Welcome to perhaps the worst webpage I have ever designed. This webpage is meant to show how perfect code and perfect design have absolutely nothing to do with each other—the code is good, the design is crap. This paragraph, for example, has an animated background and ever-changing text colour; some of those colours are hard to read against a black background.
Moving the text around is something you don't see much anymore, though in the past the marquee element was in vogue. This varies things by making it move in a rectangular pattern, but it's still hard to read. The contrast of the background isn't great either, and the animated background certainly doesn't help.
Many webpages have a background for the entire page—which is fine! But animated backgrounds should be sparingly used, if at all. Same goes with garish ones; they attract attention and if they're everywhere, the viewer won't know where to focus. That, and too many animations will actually bog down a computer.
A flashing background is a major no-no. People with photo-sensitive epilepsy may have a siezure caused by it. Besides, the text is really hard to read. Don't do it. Just... don't do it.
Three of the most important aspects of webdesign are contrast, contrast, and contrast. Contrast is the difference in colour of the text and the background. The greater the contrast, the easier the text is to read. For example, good luck reading yellow on white.
Yes, I've seen this used as a background. It sucks here, it sucked worse in that case because it was used for the whole page. While black text would help here (since most of the colours are light), the rapidly-moving background is still distracting.
An image that has strongly-different colours is especially unsuited as a background, since then you won't find a colour that will contrast well with all the colours. For example, a combination of yellow, red, and blue leaves you without a whole lot of choises—not that blue and red are great background colours to begin with. In some cases (such as this one), it can make it hard to even tell that text is there.
That the code on this webpage is flawless is meaningless; either you're highlighting the text or you looked at the mostly-unstyled webpage to see what this page actually says. Or you have better skills at reading bad websites than I do. :-)
One thing that people particularly dislike are unexpected popups. If you mouse over Paragraph 9 (this paragraph), you'll see the kind of popup that can take over a browser window. Some force you to go through a long or even endless series of such dialogue boxes, and browsers have begun implementing measures to allow you to stop the flow.