Aside from the default viewports for responsive webpages, you can also add your own viewports of any size.
To do this:
1. Enable your website's responsive layout.
2. Click the '+' icon in the centre of the Viewport Toolbar.
3. Set the width of the viewport you want to add (in this example we will use 800px, the screen resolution used by older pc computers) and then click OK. You can also click one of the common viewports to add one if you have previously removed it.
