Sure, it looks great in Internet Explorer 6, but.....
A few years ago, there weren't many different versions of web browsers to choose from. Now there are dozens. Actually, there's over 100 different browsers, not counting different versions of same product. Quite a number of these names I had never heard of before - here's a list of browsers currently available:
1X, Act 10, ActiveBrowser, Active Worlds, Alice, Amaya, ANT Fresco, Arachne, AvantGo, AWeb-II, Beonex Communicator, Browse-X, Charon, Chimera, CipherNet, Clickgarden, CrystalPort, CubicEye, DocZilla, E:ID Frame, Encompass, Enigma, Escape, ezWAP, FairLighHTML Viewer, Fast Browser, Galeon, Go.Web, Grail, Grasshopper V1, Home Page Reader, HotJava, iBrowse, IBrowser, iBrowserPlus, iCab, ICE Browser, I-Comm, iConnecter, InfoScanner, Internet Explorer, IPowerPortal WebBrowser, KBrowser MIPS Edition, KBrowser Palm Edition, Kidnet Explorer, Klondike Web Browser, K-Meleon, Konqueror, Konqueror/Embedded, Links, Lynx, Lynx for Amiga, Micro Digital Browser, Mobile Explorer, Mozilla, MSN Explorer, Multilingual Mosaic, MultiWeb
but wait, there's more!......
Neoplanet, Nestor, Netcaptor, NetClue, NetPositive, NetRaider, Netscape, Net-Tamer, Newt's Cape, Nokia Wap Browser, oKID Browser, Oligo, OmniWeb, Openwave Mobile Browser, Opera, Orangotango VirtualBrowser, Oregano, Palmscape, Pendragon Browser, Pixo Internet Microbrowser, Planetweb browser, Pocket Browser, Palm Browser, Pocket IE, ProxiWeb, Q.Bati, RapidBrowser, Safexplorer, SlipKnot, SpeedSeek Portal Solution, SPIN, StarDesktop, TV Interactor, UltraBrowser, ViOS, Voyager, w3m, Wapaka, WAPman, Web, WebsterXL, WebPhace, WebTV, WeMedia Talking Browser, Whack Force, WWW/LX, XBrowser, Yalzer, Yoozee.
Choice is a wonderful thing, but proliferation of browser software has also increased confusion in consumers as to what they should use. It also been major bugbear of web developers.
The good news for developers is that vast majority of web surfers use either Internet Explorer or Netscape - approximately 95%. The bad news is that there are over 200 flavours of Internet Explorer and Netscape. Web pages can look totally different between different versions.
The common arguement used by web developers to avoid dealing with compatibility issues is that as percentage of people using Netscape is relatively low, around 10% mark globally, - it isn't worthwhile taking those users into account. This is probably not a wise way approach issue, especially for an ecommerce based site. 10% can make a big difference to your bottom line as it's not only Internet Explorer users who buy goods and services online.
You may be of opinion "my server logs show that only 5 percent of my visitors use Netscape, so I'm not going to bother too much about cross browser compatibility". Perhaps you would have more Netscape visitors by making some minor changes to your coding that would better present your web material to them. Netscape users also have a tendency to switch to IE from time to time. If they have had a bad experience on your site using Netscape, they may not even bother visiting it again under a different browser.
Even if you do target your site to a particular brand of browser; you then need to contend with different versions of that brand. For example; a site designed for IE5.5 may look different under IE4.
The simple solution is for everyone to upgrade their browser? True, but who are we to dictate what people use on their systems? Some people do not have equipment capable to do this. The later versions of Internet Explorer demand massive system resources.
The W3C (World Wide Web Consortium) has been battling for years for software companies to produce "compliant" browsers. The idea of compliancy, amongst other things, is to guarantee that a web page looks same under any browser. Many browsers circulating now are not "compliant". Unfortunately, W3C has had an uphill battle in this but does seem to making some ground.