Drop two URLs and slide the divider
Old version on the left, new on the right. The vertical line in the middle is draggable —
move it with mouse, finger, or arrow keys ← →
(with Shift jumps by 5%, Home/End to the edges).
You can also switch viewport (Desktop / Tablet / Mobile),
measure load time for both, and copy a link
with the URLs preconfigured (great for sending to clients).
Heads up: many sites block iframe embedding (X-Frame-Options headers / CSP frame-ancestors).
If you see a blank screen instead of the page — that's why.
Browser console will show details.