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)