html, body { height: 100%; margin: 0; }

body {
	min-height: 100dvh;           /* use dynamic viewport height on mobile */
	display: flex;                /* center the image */
	align-items: center;
	justify-content: center;
	background: #fff;             /* change if you want a different backdrop */
}

img {
	max-width: 100dvw;            /* never exceed viewport width */
	max-height: 100dvh;           /* never exceed viewport height */
	width: auto;                  /* preserve aspect ratio */
	height: auto;                 /* preserve aspect ratio */
	display: block;
}