<div class="box">
  高さをCSSでheight:100%指定
</div>
html,body{
    padding:0;
    margin:0;
    height:100%
}
.box{
    background: #c1e089;
    height: 100%;
    padding: 2%;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.