The best alternatives to Responsive Web Design Tester are Froala, Responsive Viewer, and Responsive View. If these 3 options don't work for you, we've listed a few more alternatives below.
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Froala builds editing software for fast development with the better developer and user experience in mind. With over 35+ plugins and 15+ natively supported frameworks, you can plug Froala and be on your way.
A Chrome extension to show multiple screens in one view. the extension will help front-end developers to test multiple screens while developing responsive websites/applications.
Without the presence of devices, it allows you to check the display of the site layout at the same time on several screens, as well as compare the layout with design layouts. We will tell you more about the capabilities of the service in the comments.