If you want a website that works on enterprise desktops, you aren't using flexbox - you're using floats.
Are public middle schools farther along the tech curve than "enterprise"?
(Legitimately curious. It'd be pretty funny if so.)
Though there's polyfills like flexibility out there, of course.
As I understand it, IE10 implements a different, somewhat incompatible version of the flexbox spec. You can see all the changes they made for 11 here: https://msdn.microsoft.com/library/dn265027(v=vs.85).aspx