The same page at five viewports, in a row

The client asks how the hero looks on a phone. You start five browser instances on the container, one per display, each with a fixed viewport: 375×812 at 3x, 768×1024, 1280×800, 1440×900, 1920×1080. Each instance's root URL is a live display of that Chromium, so you put the five in a row of iframes on one internal page.

You paste the staging URL into all five. Resize nothing. The client scrolls the phone one while you scroll the desktop one. When they want a record, /screenshot?fullPage=true against each instance gives you five PNGs from the exact same moment.

No device lab, no DevTools emulation screenshots that look different from each other, no "let me rotate my laptop."

Responsive review stops being a window you drag and becomes a page you send.