color-mix() support

[black7375]

https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/color-mix

I'm getting an error.

    * Invalid value for `background-color` property
      syntax: <color>
       value: color-mix(in srgb, currentColor 20%, transparent)
      --------^

[skyclouds2001]

seems that it is also missing in https://github.com/mdn/data, I'll take it

[skyclouds2001]

will be added in mdn/data#857