Plenty of developers write desktop software that only works on the Mac – why aren’t more people writing web apps that only work in truly modern web browsers? The first one to do it is going to be a sensation.
I think the answer’s quite simple – it’s frowned upon. New-wave web developers still take the high road when it comes to web apps; and the progressive enhancement battalion would undoubtedly be up-in-arms over such browser/feature targeting.
I have to say, I’m with Gruber on this one. I love progressive enhancement (and write all of my Sunday morning scripts with this in mind) but if 80% of an app’s functionality can only be served up in a modern browser does it make sense from a financial or even marketing standpoint to serve the possible 20% to Internet Explorer 6 users? Obviously, the question might change if the 20% possible is the 20% of core functionality that 100% of people use.

Previous Comments ~
What I don’t get is why developers have put up with IE 6 for so long, and not made it mandatory to switch to a different browser. IE7 is a little better, but still hugely lacking, and IE 8 is what IE 6 should have been. The thing with Web Browsers is, that you can upgrade to the latest version, almost without restriction of the OS. The latest version of Opera is still available to Windows 98, so I don’t see how its so frowned upon to be forward-thinking in web design. I think the real issue is the ignorance of the average computer user.