<div class="box">
  <div class="mask"></div>
</div>
.box{
  width: 400px;
  height: 250px;
  background: linear-gradient(to bottom, red, yellow);
  position: relative;
}
.mask{
  position: absolute;
  background: rgba(0,0,0,.6);
  inset: 0 10px 20px 30px;
}

External CSS

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External JavaScript

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