It's at least as much the case that devs load up their apps with tons of third-party junk CSS and JS to smooth over the differences between old, broken versions

It's at least as much the case that devs load up their apps with tons of third-party junk CSS and JS to smooth over the differences between old, broken versions of IE and modern browsers. If we could count on people using reasonably modern browsers we could code to the standards and dispense with a lot of these clunky wrappers and workarounds.