Use the Elements tab to inspect the DOM representation of the page the browser currently displays. This will prove helpful when it comes to familiarizing yourself with the HTML code the existing widgets produce, and it will also allow you to play with classes and CSS attributes, in general. This is a great resource for testing layout changes.

The Network tab gives you an overview of which requests the current page made and how long it took. This is helpful when it comes to debugging slow page loads, as in the Network tab, you will usually find the details of the requests. If you select a request, you can inspect the payload that was passed to the server and the result returned, which helps you to figure out the reason for unexpected behavior on the client side. You'll also see the status codes of requests made—for example, 404—in case a resource can't be found because you misspelled a filename, for instance.