JSFiddle - React, Tailwind, and code Playground

HTML

<body>
	<button></button>
	
	<div></div>
	
	<button class="oneEm"></button>
</body>

CSS

html,body {
font-size:1em;
}

button, div {
	width: 3rem;
	height: 3em;
	border: 0px;
	background-color: red;
	float:left;
}

button {
    font-size:inherit;
}

div {
	background-color: blue;
}

.oneEm {
	font-size:1em;
}