<div class="box"></div>

.box {
  display:inline-block;
  width:300px;
  aspect-ratio:1;
  background:linear-gradient(135deg,#1f005c, #5b0060, #870160, #ac255e, #ca485c, #e16b5c, #f39060, #ffb56b);
  --m:
    conic-gradient(from   45deg at left,  #0000,#000 1deg 89deg,#0000 90deg) left /51% 80px repeat-y,
    conic-gradient(from -135deg at right, #0000,#000 1deg 89deg,#0000 90deg) right/51% 80px repeat-y;
  -webkit-mask: var(--m);
          mask: var(--m);
}

body {
  background: linear-gradient(90deg,#ddd,#fff);
}