HTML and CSS gracefully degrade. Host objects (DOM, etc.) require some care, but can degrade as well.
On an open platform with open software, we continue to punish and castigate users for their choice (or lack thereof) of environment. How infuriating is it for a user that's stuck on IE < 8 at work because of paranoid sysadmins?
Moreover, version detection is moronic. Twitter does this by banishing IE 6 users to a "mobile site" because the "desktop site" is poorly written. Browsing in IE 5 yields a broken version of the "desktop site" (CSS and all).