There is no Node.classList in IE9-

[SimonHarte]

You get errors in IE9 because the script uses Node.classList.add/remove. Best way to solve this would probably be to include a classList polyfill: https://github.com/eligrey/classList.js/

[MrGrigri]

Why would you use this in IE 9. Animations aren't supported. I just wouldn't even load this script if you are using anything less than IE 9.