Funny thing is, I use JavaScript libs like Ext JS because they don't impose HTML/CSS on me. I can write the whole application in JavaScript and I just have to mess with the DOM and it's ugly friends when in trouble.
doesnt matter , they use it under the hood. and the fact that you have little control over how ExtJs actually uses the DOM will not make your app faster...
That's a two-edged sword.
If I rely on an API, which abstracts everything, I benefit of every performance improvements they make "under the hood".
If they are better with performance than me, I win. If I'm better with performance than them, I lose.