<!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 id="example">Click here!</button>
		<script>
			// Set up a variable for our `button`!
			let button = document.querySelector('#example') // Any CSS selector.

			// “Listen” for clicks on the `button`.
			button.onclick = () => {
				alert('The button was clicked!') // Pop an alert!
			}
		</script>
	</body>
</html>
index.html
			html, body { height: 100%; }

body {
	--base: 20px;

	align-items: center;
	display: flex;
	font-family: sans-serif;
	justify-content: center;
	padding: 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