<div class="outer">
  <div class="box"></div>
</div>
.outer {
  width: 200px;
  background: #c0ffee;
}

.box {
  width: 100%;
  height: 40px;
  border: 1px solid #999;
  padding: 20px;
  box-sizing: border-box;
}

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.