As a web user, you might have noticed that some sites look beautifully designed, with attractive pictures and stylish typefaces, while many others look cluttered or even crude. You might also have noticed that hardly anybody uses some of the pretty ones, while some of the uglier ones are extremely popular. For example, Amazon, Craigslist, Digg, eBay, Expedia, Facebook, IMDb, MySpace, TripAdvisor, Yahoo and Wikipedia wouldn't win any beauty contests, but they're all among the most successful sites on the web.

How can this be? The answer, of course, is that most people don't go on the web to look at sites, they go to use them. They want to buy or sell something, book tickets, find information, share thinks, or chat to friends. So our key criteria are how useful and how usable sites are, which is not the same as how they look. Websites are not glossy magazines, they are software applications. Pixel-perfect graphic designs have their place, but on the web, appearance is usually less important than usability, readability, reliability, speed, and the quality of the content.

As it happens, there's a web site about usability that scores well in all these areas: Jakob Nielsen's UseIt.com. But it's not pretty, and this tends to get web designers frothing at the mouth. Nielsen is the man they love to hate.

This is a pity, because UseIt.com is where Nielsen publishes his Alertbox columns. He does usability research, writes papers, produces books and organises courses through Nielsen Norman Group, the company he runs with a former Apple Fellow, Don Norman, so his advice has plenty of commercial value. And through Alertbox, he's been giving it away since 1995.

Over the years, Nielsen has published several columns about common mistakes in web design, with the latest updated in 2007. Often these "mistakes" are obvious, and almost every web user will recognise them. But it doesn't stop countless web sites from making them.

Still, isn't it odd that Nielsen doesn't defuse criticism by updating UseIt's defiantly old-fashioned appearance? His answer is that it's a Purple Cow, which refers both to a rhyme and to a book by marketing guru Seth Godin that says: Transform Your Business by Being Remarkable.

"Keeping the same website for 10 years does make for a sort of purpleness, and that makes it hard to abandon it," Nielsen says. "Purple is not actually a great colour for a cow, but it does stand out. And in the modern world, you have to stand out.

"Redesigning it would take away the real value: I'd be just one out of 10 million. But I'm probably the only one who could get away with it."

Although some graphic designers claim they hate UseIt, ordinary users tend to like it. We usually want sites to be usable, and we get frustrated when they're not, even if we don't really know what's wrong with them. Nielsen helps us to understand the problems, and that they're not necessarily our fault.