<style>
.outer {
	float: left;
	width: auto;
	height: 100px;
	background-color: green;
}

.inner {
	width: 100px;
	height: 10px;
	background-color: red;
	padding: 10px;
	max-width: 50px;
	min-width: 30px;
}
</style>
<div class=outer><div class=inner></div></div>