This is a quick screenshot of the error message I’ve got.
First step: how to show develop menu in Safari
To show develop menu in Safari I need to go in the browser’s preferences, by clicking on Safari -> Preferences using the top menu or by simply clicking cmd + comma on the keyboard (how much I love shortcuts…)
Once done, go in Advanced field and then tick “Show develop menu in menu bar”, as shown in the picture below.
As magic we have the “Develop” menu in the top menu of our Mac. All we need to do is click on the last line, as shown in the pic below.
Third step: enjoy
Here is the final result:
If it seems not to work, try to restart Safari and try again.