You just need flex:1; It will fix issue for the IE11. I second Odisseas. Additionally assign 100% height to html,body elements.
CSS changes:
html, body{
height:100%;
}
body {
border: red 1px solid;
min-height: 100vh;
display: -ms-flexbox;
display: -webkit-flex;
display: flex;
-ms-flex-direction: column;
-webkit-flex-direction: column;
flex-direction: column;
}
header {
background: #23bcfc;
}
main {
background: #87ccfc;
-ms-flex: 1;
-webkit-flex: 1;
flex: 1;
}
footer {
background: #dd55dd;
}
working url: http://jsfiddle.net/3tpuryso/13/