block-mask-overlay-image-outset.html

<head>
<style>
.blockTest {
         margin:25px;
         padding:50px;
         color:white;
	     background-color:maroon;
         -webkit-mask-box-image: url(resources/mask.png) 75 75 75 75 / auto / 100px 0;
}
</style>
</head>
<body>
<div class="blockTest" style="height:0"><div style="height:400px; background-color:grey">This content should be masked with a soft glow.</div></div>

chromium/third_party/blink/web_tests/fast/borders/block-mask-overlay-image-outset.html