IE 10 & 11 have a number of issues with rendering flexbox properly.
Here’s one: A flex container doesn’t respect the min-height
property in these browsers.
A simple solution is to make your flex container also a flex item.
Just add this to your code (no other changes necessary):
body {
display: flex;
flex-direction: column;
}
More info: https://github.com/philipwalton/flexbugs#flexbug-3