Well, as responsivetester.net uses iframes it will not work if the site is properly protected from click-jacking using the X-Frame-Options or CSP headers, and it will also not emulate touch scrolling/events. The devtools basically bring you a little bit closer to the real thing, although you still need to test with real devices in the end either way. As for how to use it see here: https://developer.mozilla.org/en-US/docs/Tools/Responsive_De...