[css-pseudo] clarify paired cascade (#6386 + #6779 + #6774 + #7837)

[delan]

This patch makes edits for #6386:

• clarifies that “use” means used value (q1)
• clarifies that the highlight colors are ‘color’ + ‘background-color’ only (q2)
  • also clarifies that ‘stroke-color’ and ‘fill-color’ are not highlight colors
• changes the pseudos affected by paired cascade to all highlight pseudos (q3)
• clarifies that only highlight colors disable UA defaults (q4)
• clarifies that only the author origin disables UA defaults (q5)
  • also eliminates the old usage of “specified” in favour of a definition that doesn’t overload any terminology
• clarifies that ‘revert’ disables UA defaults iff the result after reverting still has values in the origins above (q6a)
  • also clarifies that ‘revert-layer’ similarly depends on the origin after rolling back the cascade
• clarifies that ‘unset’ (and other explicit defaults) disable UA defaults (q6b)
• clarifies that ‘unset’ means ‘inherit’ for all properties in highlight styles (q7)
• clarifies that ‘inherit’ also inherits parent’s highlight styles (@Loirooriol’s comment)
• clarifies that UAs are still allowed to tweak highlight colors (@delan’s comment)
  • removed for now — see [css-pseudo-4] Ensuring selection foreground/background contrast #6150, [css-pseudo] Safari’s ::selection “wash” and UA tweaks to highlight colors #6853, and @fantasai’s comment

This patch also makes edits for #6779:

• changes the effective default ‘color’ of ::{spelling,grammar}-error + ::highlight from ‘initial’ to ‘currentColor’ (p1)
• fixes the contradiction between Highlight API #default-styles and highlight inheritance (p2)

This patch also makes edits for #6774 (@delan’s comment):

• defines the inherited value of ‘color’ in the root’s highlight pseudos as ‘currentColor’ (option 2)

[delan]

@fantasai, @frivoal, some notable improvements to this patch since your last reviews:

• i’ve simplified the origins that affect paired cascade to author only (bfc6498)
• i’ve removed the text allowing UA tweaks to author highlight colors (0c46d9d)
• i’ve defined the inherited value of ‘color’ in the root as ‘currentColor’ (ce77850)
• i’ve tied the default highlight colors for ::selection + ::target-text to system colors (4eb95b0)
• i’ve rewritten and heavily simplified the new section about paired cascade itself (86bb487)

I believe the new proposed text is clearer than ever, and conveys all of the details I wanted to convey (see pr description for the complete list) in far fewer words. Please take a look!

[delan]

@fantasai, @frivoal, #6774 is now resolved, so these changes are ready for review. It looks like fantasai last reviewed 44aa6ce while Florian last reviewed 24df479, in case it helps you pick up where you left off and see what has changed.

[fantasai]

Ok, looks good. Please SQUASH and rebase before merging. :)

[delan]

Thank you both for all of your feedback!