JavaScript is a bit of a schizophrenic language, this comes about because of the fact that rather than having a javascript application operate in a stand-alone manner a javascript application is embedded in an html page.
Sadly because of this when learning to program in javascript you are sometimes left trying to figure out how html works as well, which gets especially painful when cascading style sheets are involved. Basically html was designed as a simple means by which information could be categorised as having a particular purpose, be it a heading or a paragraph, as it became very popular more and more responsibility was placed on the humble html format so that it would cope with any nutty presentation idea was thrown at it. What this resulted in was a document format that is generally over complicated and bloated for the majority of purposes, this is why I am onlly going to cover some very basic ideas here.
As most java script applications are embedded in nice feature rich web pages you have to at least have a basic understanding of how to change what the html page displays using javascript.