It lets you view any webpage in multiple screen sizes, simulating the viewport of different devices.

English Media Queries are going to establish, the pros and cons are not an issue here. "Debugging" is not easy and creating layouts can be quickly complex. This solution is an idea to know the viewport, since it shows in the upper area of the page. Via these values you can use it for different width.

‘320 and Up’ prevents mobile devices from downloading desktop assets by using a tiny screen’s stylesheet as its starting point.