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