Border by box shadow inset

You can add the border by shadow-box inset value instead of use border-box

by 3gwebtrain

HTML

<div class="parent"></div>
<div class="children">
  
</div>

CSS

.parent{
  background: #c9c9c9;
  padding:5rem;
  box-shadow: inset 0 0 0 20px rgba(0,0,0,1);
}

.children{
  padding:5rem;
  background:yellow;
}