Firefox has a convenient add-on for debugging JavaScript, CSS, HTML. Firebug supports breakpoints, watches, and has a JavaScript console editor for JavaScript debugging.
You can edit the HTML, CSS, and JavaScript documents real-time and to see how the three different pieces of code interact with each other to create the site. For more advanced users, Firebug also shows Ajax requests and responses. Interestingly enough, Firebug is written mostly in JavaScript.