[css-color-5]: Relative Colors doesn’t work with `calc(l + 10%)`

I found that this CSS doesn’t work in all browsers:

```css
.broken {
   --base: oklch(70% 0.1 30);
  background: oklch(from var(--base) calc(l - 10%) c h); /* browsers ignore it */
}
```

The only way to change lightness is to avoid `%` by using `calc(l - 0.1)`.

It looks confusing to me. But I don’t know what part of the spec prohibits it.

Demo: https://codepen.io/iskin/pen/GRabXPr