Note how the Emacs's RGB values are far off (especially the brownish shade that it thinks is bright yellow), yet the new predicate does better than all the old ones here.

> Note that AFAIR CSS (and HTML in general) uses 24 BPP colors, whereas
> your color-dark-p is AFAICT based on 16-bit color values, not 8-bit.

No, CSS and Emacs normalise hex colours to the number of digits used, so that #123, #112233 and #111122223333 all represent the same colour. (The Windows-specific colour parser has a bug, as mentioned recently on emacs-devel; I intend to fix that since other colour parsers are buggy in other ways. This is completely unrelated to the current discussion.)