<!doctype html>
<html>
	<head>
		<meta name="viewport" content="width=device-width, initial-scale=1">
		<link href="/assets/reset.css" rel="stylesheet">
		<link href="setup.css" rel="stylesheet">
	</head>
	<body>
		<button>Click here!</button>
		<script>
			// “Find” the `button` via CSS selector, then ”listen” for clicks on it.
			document.querySelector('button').addEventListener('click', () => {
				alert('The button was clicked!') // Pop an alert!
			})
		</script>
	</body>
</html>
index.html
			html, body { block-size: 100%; }

body {
	--base: 1rem;

	display: grid;
	font-family: sans-serif;
	padding: var(--base);
	place-content: center;
	place-items: center;
	row-gap: var(--base);
}

button {
	background-color: deepskyblue;
	border-radius: calc(var(--base) / 2);
	cursor: pointer; /* Show the Mickey-Mouse hand! */
	padding: calc(var(--base) / 2);
}
setup.css