Unfortunately 4.x support is still a requirement for many people, with many compromising to the point of accepting that it won't look "quite right" but won't break either, being fully functional and with formatting not obviously toasted from the user's POV.
Greenpeace, for instance, has that as a requirement for user-visible pages (with Mozilla/IE being acceptible for admin/content production pages as they can upgrade their staff's browsers).
So I'm with Simon and Dan, if we want to look like we're providing a serious web toolkit our website needs to be functional and look "OK not perfect" with 4.x.
