<!doctype html>
<html>
	<head>
		<meta name="viewport" content="width=device-width, initial-scale=1">
		<link href="/assets/reset.css" rel="stylesheet">
		<link href="style.css" rel="stylesheet">
	</head>
	<body>
		<section>
			<p>This element is responsive.</p>
			<p>I’ll add some text more here so we have something to look at and so that it wraps, but the text itself isn’t important, at the moment.</p>
		</section>
		<section>
			<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>
		</section>
	</body>
</html>
index.html
			/* Special selector that means “default”. */
:root {
	--font-size: 16px;
	--spacing: 20px;
}

@media (min-width: 500px) {
	:root {
		--font-size: 24px; /* Bump these up. */
		--spacing: 40px;
	}
}

body {
	font-family: sans-serif;
	font-size: var(--font-size); /* Use the size. */
	padding: var(--spacing); /* Use the spacing. */
}

section {
	/* The second value here is a “fallback”. */
	/* It’s used if a variable hasn’t been declared. */
	background-color: var(--background, aquamarine);
	padding: calc(var(--spacing) / 2); /* Calcs, too. */
}

section:not(:first-child) {
	--background: gold; /* Only for this element. */
	margin-top: var(--spacing);
}
style.css