Click on Safari in the browser menu, located at the top of the screen. When the drop-down menu appears, select the Preferences option.

The browser's "Advanced" Preferences will open and at the bottom is an option accompanied by a check box, labeled Show Develop menu in menu bar .

Develop should now be an available option in your Safari menu, located at the top of the screen. Click Develop. When the drop-down menu appears, select "Enter Responsive Design Mode."

The page opened in the current tab will now be displayed in rsponsive design mode. You will see a menu across the top which is self explanatory. You can switch between device displays.

Of course I am not going to leave out the Firefox users, let's go!