This script will show you a game's Steam Play community test rating(s) on the following websites, unless the game already has an official Linux version. Clicking on the results will open the game's report on https://www.protondb.com/ with the full Proton testing results.
You may also personalize the rating to your system by filtering the tests by OS, GPU driver, Proton version and max. test result age. Click on the gear icon somewhere near the rating to open up the config dialog.
The badge will be added on each Steam game's shop page either near the "buy" button (Steam), the game's title (Fanatical, ITAD) or to the game's info box (PCGW, Lutris), displaying the most frequent rating next to it or "N/A" if there are no test results yet.
An additional row to each game's info table will be added, showing how often each rating was given.
I've only tested this on Firefox with Greasemonkey 4, so if you're deviating from that and it doesn't work at all, submit an issue up top. It should also work with Tampermonkey since v0.3 though.
With Greasemonkey, the filters are stored per website instead of globally, because the GM_config script I'm using doesn't currently support Greasemonkey 4 yet.
The owner of www.protondb.com only provides data dump files anymore for the users' test results, which this script relies on. So it uses the https://protondb.max-p.me/ API to retrieve the test results instead. Because of that, they may be slightly outdated, depending on how often the dump files are updated. So make sure to check ProtonDB directly, too, especially if the rating is bad.