You don't even need to resize the whole browser window - you can use the responsive design mode of your browser's development tools to resize just the viewport of the website you are working on. And it can even emulate touch interfaces to test the relevant media queries.

And resizing the window is also something native application developers should embrace. Always roll my eyes when some game developer says they can only target 16:9 because they don't have other monitors - just make your window mode resizable and drag the corner.