Relative values like: height:100% will use the parent element in HTML like a reference, to use relative values in height you will need to make your html and body tags had 100% height like that:
HTML
<body>
<div class='content'></div>
</body>
CSS
html, body
{
height: 100%;
}
.content
{
background: red;
width: 100%;
height: 100%;
}