What distinguishes the Web sites you frequent from those you don’t, despite the similarity of their offerings? Why do you go to Books Be We, rather than Bookalocity, or vice versa? An investigation that involved more than 8,000 subjects revealed that only four important factors influence Web site popularity.

The top-scoring factor was “good content.” This is of course a great relief: it confirms that Web users are not stupid and that flashiness does not compensate for lack of content. The third most important factor was speed of downloads; and factor number four was freshness of content.

What was the second-most important reason that made users like a site? Surprisingly, it was “usability.” Surprising not because it is unexpected that users want sites to be easy to use, but because Web site builders report that whenever a company wants to cut costs of Web site development, usability is scrapped or at least postponed. Apparently, people do not realize that usability cannot be added on later. It must be built into a site (or a product of any kind) right from the start.

Many laypeople think that usability is only a veneer over a program and that it involves such issues as button placement and color choice. But in fact usability goes much deeper. Among other reasons, this is because the method of designing usability into a product involves first doing an analysis of the user’s needs, and then designing around those needs. If you haven’t done the analysis, you have to redo the design later on.

Usability involves the optimization of three factors: how quickly users can do what they want to do, how correctly they can do it, and how much they enjoy doing it. Surprisingly, sometimes these factors involve trade-offs. Researchers investigating the usability of chess game interfaces, for example, compared three different approaches: a command line interface, a mouse- based click-and-drag interface, and an interface that lets players manipulate physical pieces sensed by the computer. It turned out that the mouse-based interface was hands-down the fastest, but that players who used physical manipulation won more games-that is, they were more correct.

It has long been known that the underlying design of a computer system can affect its usability. For example, early versions of the Web’s hyper-text markup language (HTML) had “server-side image maps,” which allow the Web server to respond appropriately to clicks on different parts of a displayed image. For example, clicking on a particular country on a map of the world might take the user to information about that country.

Because information about how to respond is stored on the server, the client (that is, the user’s browser software) has no ability to give feedback about what would happen when the user clicks on the image; it cannot report, for instance, that clicking on part of an ocean would have no effect. Later versions of HTML added image maps that store the information on the client instead and can therefore indicate which areas are worth clicking on. An additional advantage of client-side image maps is that different device types can use the information to offer different interfaces: a cell phone, for instance, can let users get to the list of countries, even without a mouse.

Usability thus forms one of the touchstones around which the next generation Web markup language-called XHTML2-is being designed at the World Wide Web Consortium. Because XHTML2 is based entirely on the extensible markup language (XML), it provides an opportunity for a thorough redesign and to remove the historical flotsam that has built up over the years. Already, several pieces of XHTML2 have been published, such as the recently released electronic forms markup XForms. This improves usability by doing much more checking on the client, allowing the browser to warn the user about incorrectly filled fields before the form is submitted to the server. A draft of XHTML2 that plugs all the pieces together is now available. When the W3C is satisfied that the specification is fully implementable we will issue the final recommendation, allowing the world ‘s site builders to start using it in earnest.

This article originally appeared in the MIT Technology Insider,  a monthly newsletter covering MIT research and commercial spinoff activity.