Responsively is described as 'Web Engineering cannot get any easier!' and is an app in the Photos & Graphics category. There are five alternatives to Responsively for Mac, Windows, Linux and Online / Web-based. The best alternative is Sizzy. It's not free, so if you're looking for a free alternative, you could try Blisk or Responsinator. Other great apps like Responsively are Solis (Paid) and Polypane (Paid).
Blisk is a browser for web developers, QA engineers, and web designers with built-in mobiles, tablets, laptops, and desktops. It provides teams and freelancers with a workspace to develop and test modern web applications twice faster.
Responsinator helps website makers quickly get an indication of how their responsive site will look on the most popular devices. It does not precisely replicate how it will look, for accurate testing always test on the real devices.
Polypane is a browser built from the ground up to create websites and apps. Polypane is the only browser that lets you develop a website for all viewports and devices at once, and gives you the best overview of them. View your site at all sizes