https://alexwlchan.net/2015/11/export-urls-from-safari-reading-list/

The Developer Menu is straight-forward. Go to the Safari Preferences / Advanced and enable the menu.

The Debug Menu must be activated in the com.apple.Safari.plist. Either by issuing the following command: defaults write com.apple.Safari IncludeInternalDebugMenu 1

Or by opening the plist with the Property List Editor Application and adding a new 'IncludeInternalDebugMenu' node, with a boolean value set to 1. Same thing, different means.

Don't forget to restart Safari to see the changes.

http://127.0.0.1/transparent/

Enable Debug Menu in Safari 4.0:

defaults write com.apple.Safari IncludeInternalDebugMenu 1

via http://trac.webkit.org/wiki/WebDevelopers

Also, an interesing option is to modify the .nib file of Safari and insert a new menu: http://pointum.com/safari-tweaks.html