fix problems with ::{spelling,grammar}-error + ::highlight (w3c#6779)

Author: delan

css-highlight-api-1/Overview.bs

@@ -308,9 +308,12 @@ Applicable Properties
 
 Default Styles
 
-	UAs must not define any styles for [=custom highlight pseudo-elements=] in the default UA
-	stylesheet. A [=custom highlight pseudo-element=] inherits the styles of its
-	[=originating element=].
+	UAs must not define any default UA stylesheet rules
+	or default highlight colors
+	for any [=custom highlight pseudo-elements=].
+	In other words,
+	when some content is highlighted by an unstyled custom highlight,
+	its presentation must not change.
 
 
 Cascading and Inheritance

css-pseudo-4/Overview.bs

@@ -605,16 +605,30 @@ Default UA Styles
     :root::grammar-error  { text-decoration-line: grammar-error; }
   
 
-  For ''::selection'' and ''::target-text'',
-  the recommended default styling is to use [=highlight colors=].
-
   Note: the “paired cascade” behavior of [=highlight colors=]
   means that UA defaults for these colors
   can't be expressed in terms of 'color' and 'background-color'
   in [=UA style sheet=] rules.
   However it has been interoperably implemented in browsers
   and is thus probably a Web-compatibility requirement.
 
+  UAs may define default highlight colors
+  for each highlight pseudo-element,
+  consisting of a 'color' and a 'background-color',
+  and doing so is recommended
+  for ''::selection'' and ''::target-text''.
+  Otherwise,
+  the default highlight colors for that highlight must be
+  'currentColor' (not 'initial' or 'inherit')
+  and 'transparent' respectively.
+
+  Note: 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.
+
   UAs may apply additional effects to enhance the presentation
   of highlighted content,
   for example dimming content other than the highlighted text