Let's take a second and appreciate just how much work must go into building a popular open source JS toolkit.
Even though the domain of the problem is only JS and HTML/CSS the picture isn't as clear as you may think! Besides the technology list you also have to take into account operating system and browsers because, yes, the same browser will behave differently on different operating systems.
In addition to that, we have to also consider doctypes as they change browser rendering, and the toolkit must behave as identical as possible despite different rendering schemes.
So, as an example of the total cost of a single change as of today, September of 2008, it looks like you will have to test (assuming only major browsers and os's):
Windows: IE6, IE7, IE8, FF2, FF3, Chrome 1, Safari 3, Opera 9
MacOSX: FF2, FF3, Opera 9, Safari 3
Linux: FF2, FF3, Opera 9, Konqueror
Sixteen major browser/operating system configurations.
Not done yet:
16 configurations * 6 or so major doctypes (html/xhtml+xhtmldtd with strict/transitional and frameset versions): you end up with
~100 major variations to test after a change.
Yikes you say. Instead of rolling my own components I think I'll just choose a major js toolkit, a widely supported doctype, and learn that!
Smart choice! :)