.stack-box {
	text-align: center;
	border: solid 1px black;
	min-width: 30vmin;
	max-width: 30vmin; /* will normally be dynamically set */
	line-height: 30vmin;
	height: 30vmin;
	font-size: 7vmin;
	margin: -1px; /* make edges join neatly */
}

#input {
	width: 100%;
	font-size: 2em;
}