em is relative to its font size code example
Example: em is relative to its font size
<div id='contain' style='height: 400px'>
<div style='height: 1.5em'>Test 1</div>
<div style='height: 50%'>Test 2</div>
</div>
<div id='contain' style='height: 400px'>
<div style='height: 1.5em'>Test 1</div>
<div style='height: 50%'>Test 2</div>
</div>