Web developers cannot do without Firebug! This is a Firefox extension that lets you do a whole bunch of things. The most important of which is that it lets you easily inspect a web page and dissect all the HTML and CSS. You can even point at elements on the web page and Firebug will tell you the HTML text, the CSS tied with it, the attributes, the inheritance, etc. Or you can point at code and see the elements on the screen highlighted.
If you want to figure out how or why a page looks like how it looks... this is the solution. :)
Oh yes, Firebug does a bunch of other things too... do check it out. Google search for 'firebug'.