A simple and easy way to test your site responsiveness during customizing theme at WordPress customizer.
WordPress Responsive Customizer plugin extends your customizer preview section by adding an ability to see responsive preview of your website during theme customization. The plugin provides you dozens of different display resolutions, groupped by categories like desktops, laptops, tablets and phones. You can switch between different resolutions and see how your website will be looked on a real device. Also you can switch between landscape and portrait mode.
To use the plugin, you just need to go to your theme customizer, select desired resolution and see how your website is looked like. It's really easy to use!