fix implicit defaulting of ‘color’ at :root (w3c#6774)

css-pseudo-4/Overview.bs

@@ -704,11 +704,8 @@ Styling Highlights</h3>
   like 'fill-color'
   must not be treated as highlight colors.
 
-  The 'color' property specifies the color of both the text
-  and all line decorations (underline, overline, line-through)
-  and emphasis marks ('text-emphasis')
-  applied to the text
-  by the <a>originating element</a> and its ancestors and descendants.
+  If there is no [=cascaded value=] for 'color',
+  the [=specified value=] is 'currentColor'.
 
 <!-- Add this back if for some reason someone wants to implement 'outline'?
   The outline, if supported, must be drawn
@@ -740,17 +737,6 @@ Default UA Styles</h3>
   and doing so is recommended
   for ''::selection'' and ''::target-text''.
 
-  Otherwise,
-  the default highlight colors for that highlight must be
-  'currentColor' (<em>not</em> 'initial' or 'inherit')
-  and 'transparent' respectively.
-  In other words,
-  when the UA doesn’t define default highlight colors
-  for a given highlight,
-  highlighted content retains the 'color'
-  that would otherwise have been used
-  without that highlight.
-
   Note: the painting behavior of highlight colors
   means that default highlight colors
   can't be expressed in terms of 'color' and 'background-color'