chromium/src/third_party/blink/web_tests/fast/css/resources/image-url-var.css

.background.image-set.var {
  --test: url('test.png');
  background: -webkit-image-set(var(--test) 1x);
}
.background.image-set.inline {
  background: -webkit-image-set(url('test.png') 1x);
}

.background.url.var {
  --test: url('test.png');
  background: var(--test);
}
.background.url.inline {
  background: url('test.png');
}

.background-image.image-set.var {
  --test: url('test.png');
  background-image: -webkit-image-set(var(--test) 1x);
}
.background-image.image-set.inline {
  background-image: -webkit-image-set(url('test.png') 1x);
}

.background-image.url.var {
  --test: url('test.png');
  background-image: var(--test);
}
.background-image.url.inline {
  background-image: url('test.png');
}