I enabled the following “Advanced” Safari CSS Inspect Element trick so long ago, I forgot I had done it. Of course on my own machines, I enabled this once, and haven’t had to since. In the school MacLab though, students wanting to edit/inspect CSS will need to re-enable the Safari Develop menu each time they log in. C’est la vie.
After enabling the Develop menu, a right-click on any element of a web page should cause the Inspect Element contextual menu to appear.
There you go, infinite design possibility enabled.