I had a quick look at Percy.
In the FAQ
So, I upload screenshots?
Nope! You might be surprised to learn that under the hood, Percy is not designed to accept screenshots, but instead captures DOM snapshots and page assets (CSS, images, etc.).
They do not take screenshots! Very different from the design of other “Visual Testing” tools like Applitools. This design would enable tests on top of Percy to cover multiple devices / browsers easily. This design would be well fitting as a component of the BrowserStack service who recently merged Percy.