Preserving intermediate out of range values in relative colors is explicitly required since this commit following this comment:

[...] the whole reason we added the serialization of RCS rgb(), hsl() and hwb() to color(srgb) specifically to enable round-tripping of out of gamut values. [...] CSS Color 5 needs to specifically say that RCS versions of those color functions do not clamp out of range values.

color-mix() must also serialize with color(srgb ...) instead of rgb(). Is it also intended to preserve intermediate out of range values?

For example, what should be the resolved value of color-mix(in srgb, rgb(270 0 0), rgb(270 0 0))?

1. color(srgb 1 0 0)
2. color(srgb 1.058823 0 0)

1 is the current output in Chrome and FF (at least) and is equivalent to color-mix(in srgb, rgb(255 0 0), rgb(255 0 0)), ie. intermediate out of range values are clamped.

2 is derived from the output of new Color('rgb(270 0 0)').coords with the color-js library (which serializes as rgb(255 33.838 23.697) though).