<div class="box"></div>
.box {
  width: 300px;
  display: inline-block;
  aspect-ratio: 1.5;
  background: linear-gradient(45deg,red,blue);
  --size: 30px;
  -webkit-mask:
    conic-gradient(at var(--size) var(--size),#000 270deg,#0000 0) 0 0/calc(100% - var(--size)) calc(100% - var(--size))
}

body {
  background: pink;
}

External CSS

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External JavaScript

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