relative colors when origin is beyond gamut of output

Once the PR https://github.com/mdn/content/pull/32004 is merged, but we should address the following :

What happens if you do a relative color with the origin color being outside the gamut of the function? For example, if you use a custom property of a fantastically vibrant magenta using `oklab()` and add opacity using `rgb()` as the output. In this case, only the `alpha` value changed, but I assume the origin color is converted to `color(srgb)` rather than `rgb()`, which doesn't matter in the alpha case, but does matter if the value of `b` is changed.

_Originally posted by @estelle in https://github.com/mdn/content/pull/32004#discussion_r1525364837_