You're talking about feature testing and progressive enhancement. Adopting that strategy doesn't mean that you no longer have to test every browser, it just means that you don't support a common level of functionality across browsers. Indeed, you'll still end up testing all the browsers all of the time, just with different expectations.

These guys have been burned by IE to the point where they don't want to test it period. Feature testing doesn't get them there, but blacklisting does.