UPDATE 2025/05/04:
This site no longer works because Meta changed the format of 'achievements.json'. (I discoverd this on 2025/04/25)
Because of this change, it is impossible to see the difference between a completed achievement and an achievement in progress
Thus, impossible to detect if you have 100% full completion. Which was kinda the whole point of this website.

After the change, I searched tirelessly for a solution, and I found one (2025/05/01)
Now, a few days later, the new website is ready enough (imho) to replace this one
See you there!
https://meta-achievements.glitch.me/