[Hoss]

Careful with flexbox. It's technically stable, but very far from cross-compatible. Your best (ie9-compatible) bet would be to use a wrapper div that fills its parent, then center the form inside it with margins.

I bring with me, a solution for this aswell.

Code:

<script>
document.addEventListener('DOMContentLoaded', function() {
    if ((navigator.userAgent.indexOf('MSIE') + navigator.userAgent.indexOf('Trident')) != -2) {
        document.write('FUCK OFF');
    }
});
</script>