CSSBattle 1.1: Simply Square Solution

1<div id="outer">
2  <div id="inner"></div>
3</div>
4<style>
5  body {
6    margin: 0;
7    padding: 0;
8    border: 0;
9  }
10  #outer {
11    width: 100%;
12    height: 100%;
13    background: #5d3a3a;
14  }
15  #inner {
16    width: 50%;
17    height: 66.6%;
18    background: #b5e0ba;
19  }
20</style>