CSS data: URI CSS

:root {
  --empty-gif: url("data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==");
}

/* e.g. a transparent click target or a background placeholder */
.spacer {
  background-image: var(--empty-gif);
}