.el {
  background: ...;
  border-radius: 70px;
}
@supports (mask-image: paint(smooth-corners)) {
  .el.is-loaded {
    border-radius: 0;
    mask-image: paint(smooth-corners);
    --smooth-corners: 4;
  }
}

Note: Squircle: N=4, iOS icons: N=5