Responsive Check!! is a free Chrome add-on developed by capotast.co.jp. This tool allows users to split the current web page into two, making it possible to check the design of a responsive website on both a smartphone and a PC simultaneously.

With just one button click, the screen is divided into two windows. For responsive sites, one window can be resized to match the width of a smartphone, allowing users to easily compare the design of the PC version and the mobile version at the same time.

The width of the windows can be freely adjusted using the mouse, providing flexibility for users to view the website in different screen sizes.