html, body { margin: 0; padding: 0; width: 100vw; height: 100vh; overflow: auto; }
body { background-color: #000; }

.fullscreen-square {
	width: min(100dvw - 8px, 100dvh - 8px);
	height: min(100dvw - 8px, 100dvh - 8px);
	margin: calc(max(0dvh, 50dvh - 50dvw)) auto;
	border: 4px solid #0FF;
	display: block;
}