<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;
}
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