One mistake that a lot of new web designers make is not testing their sites in a variety of browsers. This may be an especially easy trap for Windows PC users whom all have some version or other of Internet Explorer pre-installed on their computer, but it is certainly a trap that any web-designer can fall into.

Testing in a wide variety of browsers is very important

Not everyone that visits your site is going to be using the same web-browser that you are, and your site could, much to your surprise look quite different, or even be “broken” in other browsers. If you make sure to test in a variety of different browsers then you can catch any browser specific issues with your site.

IE is in decline

Usage of IE as a whole is slowly declining, and with the exception of IE8 gaining users, individual versions of IE are all losing users. Don’t expect this to mean IE will actually disappear in the foreseeable future, but it does mean that making sure your site looks and works properly in other browsers is increasingly important.

An excellent link to check out is W3 Schools Browser Statistics.

Not all versions of IE are the same

Since there is still a pretty notable minority of people using versions of IE as old as IE6 (but only a tiny minority using even older versions), it is important that you also test your web-sites in earlier versions of IE as well.
IE is well known among web-designers for it’s various quirks that differ from one version to the next.

The easiest way to check how your site looks in several versions of IE is to download and install a program such as IETester

Should you still support IE6?

Whether anyone should even be continuing to support IE6 or not is also a matter of some debate, with some designers choosing to support it either by limiting what features they use to ones IE6 can handle, or using various hacks, and others choosing not to. One option is to use JavaScript to send IE6 users a message that they are using an out of date browser and should update.

Test In more standards compliant browsers first

Instead of jumping in and dealing with IE’s quirks right away, I recommend testing sites in a browser such as FireFox or Opera first, and deal with the particular quirks of the various versions of IE afterward. This isn’t to say you’ll never encounter some odd quirk in FF or Opera, but IE is certainly among the most quirky, and if you design a site for IE, you may find it’s more of a headache to make it work decently in pretty much any other browser, than it would be if you did the reverse.

Put your site through the W3C Validator

While some invalid code, at least in current browsers, may not end up causing you any grief, using non-standards complaint code is a common cause of websites looking “off” in a browser (or browsers), and can cause other problems as well.

I recommend going to W3C’s Validator and checking for errors in your code.