Forced Colors Mode is difficult to test via WPT because it is dependent on OS state (Window High Contrast) to trigger the feature.

This proposal is to add a new WebDriver extension command that keeps track of the current forced colors mode emulation state (either "none", "light", or "dark"). The Forced Colors Mode spec text would then need to be updated for UAs to force Forced Colors Mode if this state is not "none" and change the mappings of system color keywords based on the emulation theme to a set color palette.

I have put together a rough draft of how this could look in #11823. I used the Win 10 Black and White themes for "light", or "dark" color mappings, but these could be any colors we'd like.