Something I'm seeing over and over in this forum is something to the effect of: I'm trying to do X, and it works in [browser1] but it won't work in [browser2], what is wrong?
The point of those libraries is to abstract away the nuts and bolts of getting a particular DOM object from a given browser, or changing values around. They do all the work of making your code browser cross compatible.
I personally use JQuery, which I can't recommend highly enough, but any of the major libraries will server you far better than raw DOM scripting. You will be more productive, and less frustrated, I promise.