I'm not sure I understand what you're asking. Could you elaborate and provide some more context please?

What color-space will dropper color be reported in? (eg. imagine an image loaded from a different space to the screen)

Can I round trip the color? eg. sample <div background-color:#123>, get the color from the dropper, set the color on the div, will its color change even slightly?

I'm not sure which dropper you're referring to. Could you point me to the relevant specification?

In case it helps, hex colors (like your #123 example) on the Web are in the sRGB color space.

As discussed in TAG breakout today, we think this is probably redundant with #587 so closing this one for now. Please move conversation there. Thanks!