As others have stated, there is no point in a "reveal". Simply show the comparison upfront. User interaction should not be necessary as it only needs to be a plain old html document. You didn't need to write any script. When I got through a bit of it, I wasn't sure if I accidentally skipped something or missed a nuance.
Maybe `ArrowLeft`/`ArrowRight` or `j`/`k` to flip through all of card pairs? Or just allow us to scroll through all the pairs. In either case, both old and new should be visible.
I would love to just scroll through a page without having to click on individual cards to see both side by side.
Love the overall idea of what you did!