Mootools, as probably every other JavaScript application or framework has added prototypes to built-in objects o JavaScript but when ES6 added prototypes with the same name (but with different functionality), everything broke.

- JS prototypes String.prototype.constrains() ES6
- with
- Array.prototype[@@unscopables]
- DOM window.orientation - there was no spec for it... undefined != 0
- CSS
- showModalDialog()
- AppCache
- isindex
- Read web standards!
- test your website in other browsers as well
- Report bugs at webcompat