Sandbox

style.css
setup.css
index.html

div {
	background-color: aquamarine;
}

@media (min-width: 400px) {
	div:first-child {
		background-color: coral;
	}
}

@media (width: 400px) {
	div:nth-child(2) {
		background-color: orange;
	}
}

@media (max-width: 400px) {
	div:last-child {
		background-color: gold;
	}
}

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

div {
	padding: 10px;
}

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

<!doctype html>
<html>
	<head>
		<meta charset="utf-8">
		<meta name="viewport" content="width=device-width, initial-scale=1">
		<link href="../assets/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>
		</div>
		<div>
			<p>This is another responsive element, with 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>
		<div>
			<p>And a third one, to have an element for each rule. I probably should just grab some lorem ipsum, but at this point I’m committed to typing nonsense.</p>
		</div>
	</body>
</html>