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