[jpedrosa]

For the most part, several Javascript libraries have solved the basics of cross-platform browser support quite well and entire ecosystems were built using them.

I regard Ext Core to being quite similar to other libraries even though it could pack more quality features into just a relatively small file. That is, If I were looking for something "current" I would look into Ext Core as well. But I have high hopes for YUI 3 which will have dynamic loading of features and contextual programming with both CSS and Javascript to let folks create relatively safe and packaged Javascript programs that could then be embedded in other sites without disrupting their looks or functions.

Perhaps YUI 3 is a little bit on the "over-engineering" side, but so long as it works as a stable foundation down the road it could well be the future.