If the problems with that "crap" were insurmountable we wouldn't have the huge variety of websites like we do today. And I would argue that just as big cross platform issues exist with Qt/Swing.
With Modernizr, JQuery etc you can get 95+% of the features on 95+% of computers.
With Modernizr, JQuery etc you can get 95+% of the features on 95+% of computers.