CSS百分比

demo

HTML

<div class="border-container">
<div class="cube">测试一下</div>
</div>

CSS

.border-container{
  width: 100px;
  border: 1px dotted #666;
}

.cube{
  width: 100px;
  height: 100%;
  border: 1px dotted #999;
  position: absolute;
}