[basementcat]

I think this depends on who your customers are. If your customer is a regulated utility that may be required to use equipment that has been "licensed" for certain purposes, making sure websites work with older versions of browsers is very important. On the other hand, if you're targeting high-ish net worth end users who may be expected to always run the most recent browser version on the most recent hardware, then it would make sense to always target the most recent features.