On both Firefox and Chrome, when I hover over either of the videos, the thin dividing line disappears, and it's not clear which video I'm looking at. Albeit they are "supposed to" look very similar, I still don't know whether I'm watching the original or the compressed one.

Typically in similar sites, I've seen a draggable divider that lets the user specify how to horizontally divide the frame between the two comparable videos.

Do you process an included audio track as well?