mask-composite-source-in-single-layer.html

<!DOCTYPE html>
<style>
#target {
  width: 100px;
  height: 100px;
  background-color: green;
  -webkit-mask-image: linear-gradient(white, white);
  -webkit-mask-composite: source-in;
}
</style>
<div id="target"></div>

chromium/third_party/blink/web_tests/css3/masking/mask-composite-source-in-single-layer.html