Undoing Element Expanding in CSS

As a rule, the expansion described in the previous lessons is harmful and you would like to get rid of it. For this purpose, CSS has the box-sizing property. If you specify the value border-box for it, then neither padding nor the border will expand the block:

<div id="elem1"></div>
<div id="elem2"></div>

#elem1 {
	width: 100px;
	height: 100px;
	background: #f1f1f1;
	margin-bottom: 20px;
}

#elem2 {
	width: 100px;
	height: 100px;
	background: #f1f1f1;
	padding: 25px; /* set indents */
	border: 10px solid green;  /* set the boundary */
	box-sizing: border-box;  /* remove extension */
}

Try the described box-sizing property yourself.