If lab() function is supported, the box is pink, else it's blue.

At the moment of writing, only Safari support lab().

@supports not (background: lab(0% 0 0)) {
  .box {
    background: lightblue;
  }
}

@supports(background: lab(0% 0 0)) {
  .box {
    background: lab(80% 100 50);
  }
}

@supports (display:flex) {
  .box {
    display: flex;
  }
}