<div class="box">The width of this box is 440px, even though it is defined at 400px in the CSS.</div>
.box {
  margin: 0 auto;
  background-color: aquamarine;
  width: 400px; 
  height: 400px;
  padding: 20px;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

  1. //cdnjs.cloudflare.com/ajax/libs/jquery/2.1.3/jquery.min.js