I think you just need to give it a parent container... even if that container's height is defined in percentage.
This seams to work just fine: JSFiddle
html, body {
margin: 0;
padding: 0;
width: 100%;
height: 100%;
}
.wrapper {
width: 100%;
height: 100%;
}
.container {
width: 100%;
height: 50%;
}