Check a layout across four, six, ten viewports at once — synced scroll, clicks and input — without opening a separate desktop app or paying a monthly seat.
The strong competitors in this space are standalone desktop apps — which means a second cookie jar, a second login, a second window to keep in sync with what you're actually building. ResponsiveX runs where the work already is.
Built on the domain logic of the open-source Responsive Viewer, with the platform layer rebuilt for Manifest V3 and three gaps closed that its users kept filing issues about.
ResponsiveX is distributed directly for now — Chrome won't install a downloaded .zip in one click, so it's a short manual load:
Download the .zip above and unzip it anywhere on disk.
Open chrome://extensions in a new tab.
Turn on Developer mode (top-right toggle).
Click Load unpacked and select the unzipped folder.