If you dont use Internet Explorer for Windows, you are on the margins of the internet. Its kind of like being a non-American in our unipolar world. The mighty behemoth, IE does what it likes. Standards be damned, its practices are the de facto standards which matter.
Im in charge of a web site and know first-hand how tedious it is develop a site which works tolerably well for almost all browsers, without being intolerably bland. I recently had a discussion/argument with a respected colleague about this issue. He said that there are so many different permutations of browsers, that getting our sites to work for all of them is an exercise in futility which leads to the dreaded Lowest Common Denominator. For me, this is not an all or nothing matter. I try to support what I subjectively think are the main types of people on the web and the browsers they often choose.
Theres Mr and Ms Average people who use IE for Windows, 5.x and 6.x the browser that everyone uses when they dont have a choice (or exercise choice) in what browser they use. Needless to say, its got to work very well for this browser. I admit, some people might consciously choose this solid but boring browser. For me, I only use this browser when none of the others works or I might to use some third-party software which only works with IE such as the Google toolbar.
You have to acknowledge the technical laggards of the world people using IE and especially Netscape 4.x. Im not saying that a site has to work well for these people, but see how the home page looks and hope that the site is functional. Sometimes minor tweaks in the code will improve performance in these oldies without mucking things up for everyone else. If thats not possible, use a script to redirect them to a text-only of your home page. This text-only version could also benefit the visually impaired who are using a text-only browser.
You also cant ignore people who are bravely (or stupidly, depending on your point of view) using non-Microsoft software namely Mac and/or Linux users. I dont know where the market share statistics come from, but I work in education and see a lot of Macs around and talk to a lot of people who are passionate about Macs. So even if you are a hard-nosed Windows-head, you dont want to needlessly piss these persistent people off, because they can be vocal in their criticism. If nothing else, make sure that the site works in IE 5.2 or Safari. If you dont have access to a Mac, send the URL to a friend, acquaintance or relative who has a Mac and ask for feedback. The same goes for Linux, make sure that it works for at least one Linux browser, especially if you know that Linux people will be using your site.
Another small but influential group are the geeks who may be using alternative browsers such as Mozilla or Opera. They may very well be your peers. They are also the people who often look at web pages critically. You dont want them to be sending you emails saying that your web site isnt working for them.
At the other extreme are the AOL users. Although Windows AOL is based on IE, it is not identical. AOL for Mac OS X is based on Netscape 7 for OS X. Although testing maybe difficult for AOL, if you become aware of a problem, try to find out what it is and how it might be fixed. Because there are still a lot of people using AOL as their browser.
All this sounds like a lot of extra work, but the only significant task is to get out of the IE 6.x for Windows or bust mindset. Because once you start testing your website on a few different browsers, youll get a feel for what works and what doesnt work on the other browsers.
These are only suggestions for the minimum. Because I am a Mac head, Im going to make sure that my web sites dont work well for just one current Mac browser, but for as many as possible.