Safari has long had a hidden Debug menu that contained useful commands. Originally intended to help developers debug web pages and the JavaScript code that runs on them, the Debug menu was hidden because commands listed in the menu could break web pages. The information in this article applies to Macs running OS X El Capitan (10.11) or earlier.
How to Enable Safari's Hidden Debug and Develop Menus
With the release of Safari 4 in the summer of 2008, many useful menu items in the Debug menu were moved to the new Develop menu. The hidden Debug menu remained, however, and even picked up a command or two as Safari development continued. It was removed entirely from OS X Sierra and later releases of the operating system.
Apple made opening the hidden Develop menu a simple process, requiring only a visit to Safari’s preferences. Opening the Debug menu, on the other hand, was a bit more complicated.
To enable the Safari debug window, you need to use Terminal, a powerful tool for accessing hidden features of the Mac operating system and its many apps. Terminal is the secret to enabling the Safari Debug menu.
