Preview Website Pages
After opening the website editor sidebar, below the page title there are editing controls. These include several preview icons to assist with viewing the site layout with different sizes such as your current browser, mobile phone, tablet, and desktop.
Examples of the same (test) site sections:
Browser - essentially hides the website editor sidebar.
Mobile - shows a common portrait layout emulating a mobile telephone view in a pop-up window.
Tablet - shows a common landscape layout emulating a tablet view in a pop-up window.
Desktop - shows a standard view as seen in most desktop browsers in a pop-up window.