Keyframe including invalid code is removed while browsers ignore the invalid code part

[sapphi-red]

@keyframes gradient-success-in {
  0% {
    --tw-gradient-from: color-mix(
      in oklab,
      oklch(59.6% 0.145 163.225) 0%,
      transparent
    );
    @supports (color: color-mix(in lab, red, red)) {
      --tw-gradient-from: color-mix(
      in oklab,
      var(--color-emerald-600) 0%,
      transparent
    );
    }
  }
  100% {
    --tw-gradient-from: color-mix(
      in oklab,
      oklch(59.6% 0.145 163.225) 20%,
      transparent
    );
    @supports (color: color-mix(in lab, red, red)) {
      --tw-gradient-from: color-mix(
      in oklab,
      var(--color-emerald-600) 20%,
      transparent
    );
    }
  }
}

is transformed to

@keyframes gradient-success-in {}

.
While the input is invalid, I expect lightningcss to output:

@keyframes gradient-success-in {
  0% {
    --tw-gradient-from: color-mix(
      in oklab,
      oklch(59.6% 0.145 163.225) 0%,
      transparent
    );
  }
  100% {
    --tw-gradient-from: color-mix(
      in oklab,
      oklch(59.6% 0.145 163.225) 20%,
      transparent
    );
  }
}

because browsers (tested with Chrome) treats the input like that. Also it would be nice to have a warning.