<section class="parent">
	<div class="fixed-child-content">
		<div class="inner">
			This is position fixed content.
		</div>
	</div>
</section>
.parent {
	max-width: 300px;
	width: 100%;
	margin: 0 auto;
	background: #ccc;
	height: 100vh;
}

.fixed-child-content {
	position: fixed;
	background: #000;
	color: #fff;
	bottom: 0;
	max-width: inherit;
	width: 100%;
}

.inner {
	padding: 20px;
}
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