Rune's version of color-scheme gives "light" as the initial value, and assumes "light" if the user doesn't express a preference.

This is definitely what's required for web compat in general, but there are still minority user agents that do not render with a "light" style by default (and just accept that they're not always web-compatible).

I propose adding an "auto" value to 'color-scheme' to act as an initial value instead, indicating that the author has no idea what color schemes are, and so the UA should use the legacy "default" color scheme ("light" for maximum web-compatibility). This is currently in the spec, so you can see what it looks like.