html, body {
  height: 100%;
  margin: 0;
  overflow: hidden; /* no scrollbars */
  background: #000; /* behind image */
}

img.fullscreen {
  width: 100vw;
  height: 100vh;
  object-fit: cover;     /* fill screen, crop if needed */
  object-position: center;
  display: block;        /* remove inline gap */
  user-select: none;
  -webkit-user-drag: none;
}

/* Avoid mobile browser address-bar jump where supported */
@supports (height: 100dvh) {
  img.fullscreen { height: 100dvh; }
}