<div class="box">
	<div class="em">EM</div>
	<div class="rem">REM</div>
</div>

html {
	font-size: 15px;
}
.box {
/*   font-size: 20px; */
	color: white;
	display: flex;
}
.em{
	width: 10em;
	height: 10em;
	background-color: red;
}

.rem{
	width: 10rem;
	height: 10rem;
	background-color: blue;
}