Yahoo User Interface toolkit

YUI is a good compromise; it has a full set of widgets but does not suffer from some of the problems of other Javascript MVC frameworks:

• it does not do away with HTML; markup can be used to define many of the widgets so web designers can be included in the development process.
• the download overhead is not not so large.
• it is much more page oriented than many Ajax frameworks (thanks partly to the smaller download).
• it has good documentation.
• it is backed by a big player.