I'm trying to move to CSS only, however I have a couple of stumbling blocks to overcome. First, what's the difference (advantages?) of using CSS id vs CSS class (#content vs .content for instance...).

As for it's importance. Well, maybe it has something to do with optimizations? (well, once you've found an id, there's technically no need to look for it anyfurther). Also, id's weigh alot in the ...