difference between $('html, body').animate and $('body').animate?

The reason you use a selector for BOTH $('html, body') is because of web browser inconsistency. After a few tests I have found three things:

1. The browsers Firefox & IE utilize the html portion of this selector
2. Browsers in the "webkit class" eg: Safari and Chrome respond to the body.
3. Although one can avoid the issue all together by using $(document) instead.

There's also a ticket on the jQuery bug tracker specifically stating this issue here