But your site can be broken at different sizes even without breakpoints. From a QA perspective with manual testing, there's no difference whether or not develop

But your site can be broken at different sizes even without breakpoints. From a QA perspective with manual testing, there's no difference whether or not developers have used breakpoints, the question is whether the application is working for the users' screen sizes. And, as the article points out, they are myriad. To test that the responsiveness works everywhere, the easiest thing is to probably go to the "responsive" tool in the browser's devtools, which opens the page in a little subwindow inside the main browser window, and then you can resize the subwindow freely to check a range of different sizes (rather than just an explicit set of them).