The Web Inspector!
I remember years ago when I first studied HTML, the "View Source" feature was the way that I studied. If I wanted to know how someone had done something, I'd check out the source. Of course, nowadays, all the websites use scripts and CSS to achieve what you see on screen. "View source" is now nearly useless. The new Develop menu in Safari has something that I think is pretty cool however. It is the Web Inspector. It does pretty much what "View Source" used to do. It separates everything that is on the website into the following categories: Documents, Stylesheets, Images, Scripts and Other. You can find the Web Inspector in the Develop menu. The Develop menu can be enabled in the Advanced tab of Safari's preferences.