Demo

index.html
script.js
setup.css

<!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">
		<script defer src="script.js"></script>
	</head>
	<body>
		<button id="example">Click here!</button>
	</body>
</html>

// Set up a variable for our button!
let button = document.querySelector('#example'); // Any CSS selector.

button.onclick = () => { // Attach the click event.
	alert('The button was clicked!'); // Pop an alert!
};

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);
}