In today's multi-device world, responsive web design has become essential for creating websites that adapt seamlessly to various screen sizes and r...
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You should mention, that modern browsers have integrated test facilities for responsive design. In chrome you open the DEV-tools pressing F12. Here you find the responsiv layout tests (second icon from the left):
The test panel gives you numerous predefined screens, but you can also check you site with changing width. It even allows to simulate a slow device.