width: 100%

HTML

<div class="outer">
  parent
  <div class="inner1">width: 100%</div>
  <div class="inner2">width: auto</div>
</div>

<ul>
 <li><a href="https://teratail.com/questions/158834#reply-238160">CSS - width: 100% と、width: auto の違い｜teratail</a></li>
</ul>

CSS

div.outer {
  border: solid 1px red;
  padding: 1em;
  background-color: #fee;
}
div.inner1 {
  width: 100%;
  margin: 1em;
  padding: 1em;
  border: solid 1px green;
  background-color: #efe;
}
div.inner2 {
  width: auto;
  margin: 1em;
  padding: 1em;
  border: solid 1px blue;
  background-color: #eef;
}