There is one tool that I use the most for web design and that is Firebug. It is a simple firefox plugin that lets you dive into the CSS and HTML of a web page to see how it is built and works. It is great for understanding CSS and working out my stuff isn't working as it should be.
What firebug offers is a simple interface pane that appears on the bottom of the screen. You have on the left the html of the webpage, this is used in flippy triangle format so you can collapse great panes of html if you don't need it. On the right you have the CSS this is live, so you can change the CSS and see what happens.
It is a great tool to see how pages work, you can also fiddle about with pages to see why they aren't working as they should be. As well as this they also include script messages, as well as DOM and Net errors. A try developers tool. I recommend it to any designer new or old.