This is a nice goal, especially if you need portability all the way down (... to IE 5.5 or something). However, for any larger web application, this increases

This is a nice goal, especially if you need portability all the way down (... to IE 5.5 or something). However, for any larger web application, this increases the testing effort dramatically, as you need to test all possible sets of capabilities (i.e. all kinds and versions of browsers and other clients). This becomes quickly unbearable burden, unless all functionality is 100% provided and battle-tested by the framework.