Demo

style.css
setup.css
index.html

div { background-color: aquamarine; }

aside { background: gold; }

@media (hover: hover) {
	aside { visibility: hidden; }

	div:hover + aside { visibility: visible; }
}

body {
	font-family: sans-serif;
	padding: 20px;
}

aside,
div {
	padding: 10px;
}

aside,
div:not(:first-child),
p:not(:first-child) {
	margin-top: 10px;
}

<!DOCTYPE html>
<html>
	<head>
		<meta name="viewport" content="width=device-width, initial-scale=1">
		<link href="/assets/styles/reset.css" rel="stylesheet">
		<link href="setup.css" rel="stylesheet">
		<link href="style.css" rel="stylesheet">
	</head>
	<body>
		<div>
			<p>This element is responsive. I’ll add some text here so we have something to look at and so that it wraps, but the text itself isn’t important, at the moment.</p>
			<p>Some more text in it. Again the text doesn’t really matter, I just want a bit of stuff in here. A few lines, is all.</p>
		</div>
		<aside>This is some really important text that you want to make sure mobile users can see, too!</aside>
	</body>
</html>