Edit in JSFiddle

<div class="parent">
      <div class="em-demo">
		  设置长度为5em demo
      </div>
		<div class="percentage-demo">
		  设置长度为80% demo
		</div>
    </div>

div{
	border: 1px dashed #808080;
	margin:10px
}
.parent{
	width: 200px;
	font-size: 18px;
}

.em-demo{
	width: 5em;
    font-size: 1em;
}

.percentage-demo{
	width: 80%;
    font-size: 1.5em;
}