fix bug where explicit defaults don’t exist in specified-value space

css-pseudo-4/Overview.bs

@@ -766,40 +766,54 @@ Highlight Colors</h4>
 
 In [=highlight pseudo-element=] styles,
 'color' and 'background-color' are known as <dfn>highlight colors</dfn>.
-For compatibility reasons,
-UA default highlight colors must only be [=used value|used=]
-when neither of the highlight colors yield a [=specified value=]
+
+With the exception of ''revert'' (see below),
+UA default highlight colors
+for a given [=highlight pseudo-element=]
+must only be [=used value|used=]
+instead of [=initial values=]
+when neither the originating element’s [=cascade=],
+nor those of any of the originating element’s ancestors,
+yield any highlight colors
 of the [=author origin|author=],
 [=animation origin|animation=],
 or [=transition origin|transition=] origins.
 In other words,
-when setting one custom highlight color,
+when setting one “custom” highlight color,
 UA defaults for the other highlight color are suppressed.
 
 This does not preclude the UA
-from transforming non-default highlight colors
+from transforming “custom” highlight colors
 in some or all situations.
 For example,
 UAs may invert highlight colors based on similarity with nearby colors,
-or vary their opacity.
+or vary their opacities.
 
 Note: 'stroke-color' and 'fill-color' have their usual effects
 on the foreground color,
 but unlike 'color',
 they do not suppress the UA default 'background-color'.
 
-For the purposes of this section,
-''unset'' is considered equivalent to a [=specified value=] of ''inherit''
-in the [=declared value|declaring=] origin.
-This makes the value not quite equivalent to
-erasing all [=declared values=] occurring earlier in the cascade,
-as would normally be the case.
-
-On the other hand,
-when a highlight color is ''revert'',
-the origin of the resultant value
-<em>after</em> rolling back the cascade
-is considered by this rule.
+When a highlight color is ''revert'',
+UA default highlight colors are suppressed
+if and only if the cascade is “rolled back”
+to one of the origins listed above.
+For example,
+''revert'' in an [=author-origin=] rule
+rolls back the cascade to the [=user origin=],
+so UA defaults are not suppressed.
+
+Note: all other explicit defaults,
+including ''unset'',
+suppress UA default highlight colors like any ordinary value.
+This is consistent with
+how [=explicit defaulting=] is defined:
+it affects [=specified values=]
+based on [=cascaded values=].
+Even though the outcome is <em>as if</em>
+the [=cascade=] was modified in some way,
+the [=cascade=] is already done,
+so they exist unchanged as [=cascaded values=].
 
 <h4 id="highlight-backgrounds">
 Backgrounds</h4>