To enable the Safari Develop menu, just close Safari and type (or copy/paste) the following command into Terminal.app (/Applications/Utilities):
defaults write com.apple.Safari WebKitDeveloperExtras -bool true
Once you enter the command, you will be able to launch Safari, load a webpage, and click Develop > "Show Web Inspector" to see the page attributes. You will be able to see the documents, stylesheets, images, and scripts.
Update: As of new versions of Safari, you can enable the Develop menu by going to Safari > Preferences > Advanced, and click on the "Show Develop menu in menu bar." Thanks to everyone in the comments for pointing this out!
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