IE has another issue, that of testing. A Windows machine will only ever want to have one version of IE installed at a time. If you’ve updated to IE9, you can’t keep a copy of IE6 on your system. By contrast, installing multiple versions of Firefox or Safari is a relatively straightforward process. (Not that doing so is usually necessary: the upgrade rate for non-IE browsers is fast, largely relieving you from the necessity of supporting old versions.)

The situation for IE is not quite as bad as it sounds, as IE8 has an “IE7 emulation mode” added to try to avoid breaking sites that were crafted to earlier versions of the browser that did not follow standards.

The ideal solution is to have several different computers running different versions of Internet Explorer: IE8 (with the option to emulate IE7) on one computer, and IE6 running under Windows XP on another machine, both looking at the same page. Alternatively, you could have browsers running on different virtual machines.

Increasingly the favoured solution is to use an online emulator, such as Adobe’s BrowserLab, browsershots.org or browserstack. Generally speaking these emulators are easier to use than installing a suite of browsers on your own machine; they can also be used from any computer with an internet connection, and provide a wide coverage of different browsers and platforms. However, being internet services, rather than local programs, they are also somewhat slower in providing feedback. As a result, anticipating and fixing as many problems as you can in advance becomes important (as opposed to working on problems line-by-line and waiting for the online emulator to respond in order to check each one).