I get your point but there is one problem with that approach that has lead me to advise some clients to use such a filter. People like most of us reading here know quite well the evils and shortcommings of IE6, but by far the most people still using IE6 don't. I know it is hard to imagine for anyone here but I have friends whom I had to explain that some non-functional website was due to their prehistoric browser. Telling the public their browser is way too old is far better than presenting them a partially functional non-marked up website. It may annoy you and me, but we're not the public most webdesigners should care about.

Now to our own site disallowing IE7 while being targetted at web professionals. Pretty much no-one doing serious web development will be limited to IE6/7 - let alone use it as their primary browser. If you do and your employer is short-sighted enough to keep at IE6, you'll be part of a very very tiny minority which will most likely still not have access to PHP5.3 either.

Sigh. To avoid further downvoting, I may add that the 'blank css' approach makes more sense than the 'go away' one.

But I was thinking of things like the different browser (HTML5) Audio APIs 'we' are facing right now and my point was that you are probably better off just focusing on getting things right in one environment first before spending too much time on 'making it work for everyone'. Which is 'overrated' anyway, depending on the project. Just think of how Beatport.com is entirely based on Flash which is still the most convenient way of adding audio.