width 100%

by ud25tGNwMddCP7JI

HTML

<div class="parent">
  <div class="my-border">

  </div>
</div>
<br>
<div class="parent">
  <div class="my-border">

  </div>
</div>

CSS

.parent {
  width: 500px;
  height: 150px;
  border: solid 10px black;
}

.my-border{
  border: solid 20px red;
}

.my-blue {}

.my-green {}