Uncaught TypeError: document.attachEvent is not a function

The conditional comment needs to be

<!--[if lt IE 10]>
  <script src="flexibility.js"></script>
<![endif]-->

Flexbox doesn’t work in IE9 so it needs to be loaded there as well. Or it could even just be this since conditional comments don’t work in IE 10

<!--[if IE]>
  <script src="flexibility.js"></script>
<![endif]-->

The fix is good to have 👍

It would be nice to have a conditional comment version in the readme. It’s a bandwith/connection saver.

<!--[if lte IE 9]>
  <script src="flexibility.js"></script>
<![endif]-->