Site Editor's built-in responsive design resizes your content and images to fit different devices and screen widths.
You can preview the site you are building as it will appear on a Desktop, Laptop, Tablet and Phone from both the Site Editor and the Preview Mode in Site Editor with the help of the Device View controls: click the control to switch the View Mode.
In Edit Mode, the Device View controls are located at the top of the screen: In Preview Mode, the View controls are located at the bottom of the screen.
In both modes, Edit and Preview, you can edit the site. In Preview mode, editing is limited: you can only edit elements of the layout (text, headings, photos, buttons) from the editing menu that appears once you select the element.