If you use general selectors in your css it could potentially override my css.
Eg, if you use .ui-widget input, and if im trying to style an input in a widget it means I have to put !important on all my css. this might not be enough as your css might have any attribute which I will also need to include in my css.
so it you use specific selectors, eg, .ui-widget-input it would avoid the problem. I think?
I realise if means you will need to write a little more code, but surely the goal of a library is to not cause side effects to the user of the library.