Array.prototype.flatten not Web-compatible because of old versions of MooTools

You'd think that this was an April Fool's joke. I hope that there are enough sensible members on the committee to reign in the emotional immaturity of their younger members.

High level TL;DR:

A new JavaScript spec is proposing adding a flatten() method to Arrays. An old JavaScript library that some websites use however also uses a flatten() method on arrays, and browsers supporting the new spec would cause those old websites to break.