Browser support
Like jQuery itself, jQuery UI supports all of the major browsers in use today including the following:
IE6, IE7, and IE8
Firefox 2 and Firefox 3
Opera 9 and Opera 10
Safari 3 and Safari 4
Chrome 1, Chrome 2, and Chrome 3
This is by no means a comprehensive list, but I think that it includes the most common browsers currently in use, making jQuery UI very well supported.
The widgets are built from semantically correct HTML elements generated as needed by the components. Therefore, we won't see excessive or unnecessary elements being created or used.