[css-pseudo] clarify paired cascade (#6386 + #6779 + #6774 + #7837) (#6665)

Author: delan

css-highlight-api-1/Overview.bs

@@ -319,9 +319,12 @@ Applicable Properties</h4>
 <h4 id=default-styles>
 Default Styles</h4>
 
-	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 <a spec=css-pseudo>default highlight colors</a>
+	for any [=custom highlight pseudo-elements=].
+	In other words,
+	when some content is highlighted by an unstyled custom highlight,
+	its presentation must not change.
 
 <h4 id=c-and-h>
 Cascading and Inheritance</h4>

css-pseudo-4/Overview.bs

@@ -696,7 +696,8 @@ Styling Highlights</h3>
 
   Issue: Are there any other properties that should be included here?
 
-  The 'color' property specifies the color of both the text
+  Note: The 'color' property sets the color of
+  both the text
   and all line decorations (underline, overline, line-through)
   and emphasis marks ('text-emphasis')
   applied to the text
@@ -724,16 +725,68 @@ Default UA Styles</h3>
     /* Represent default spelling/grammar error styling in an adjustable way */
     :root::spelling-error { text-decoration-line: spelling-error; }
     :root::grammar-error  { text-decoration-line: grammar-error; }
-    /* Highlight targeted text */
-    :root::target-text    { color: MarkText; background: Mark; }
   </pre>
 
+  Some [=highlight pseudo-elements=]
+  should have <dfn>paired default highlight colors</dfn>--
+  a default 'color' and 'background-color'
+  provided by the UA
+  that are either used or overridden together,
+  see [[#paired-defaults]].
+  For ''::selection'' they should correspond to
+  ''HighlightText'' and ''Highlight'',
+  while for ''::target-text'' they should correspond to
+  ''MarkText'' and ''Mark''.
+
   UAs may apply additional effects to enhance the presentation
   of highlighted content,
   for example dimming content other than the highlighted text
   or transitioning out a highlight style based on user interactions or timing.
   These are not controlled by CSS.
 
+  ISSUE: UA tweaks to the presentation of highlights
+  in ways that <em>are</em> controlled by CSS
+  are currently under discussion
+  in <a href='https://github.com/w3c/csswg-drafts/issues/6853'>Issue 6853</a>.
+
+<h4 id="paired-defaults">
+Paired Defaults</h4>
+
+  For compatibility reasons,
+  [=paired default highlight colors=]
+  must only be [=used value|used=]
+  when neither 'color' nor 'background-color'
+  yield a [=cascaded value=]
+  from the [=author origin=]
+  (or inherit their value from the author origin).
+  When a highlight color is ''revert'' or ''revert-layer'',
+  the origin <em>after</em> rolling back the cascade
+  determines the [=cascaded value=]’s [=cascade origin|origin=].
+
+  Note: Because this rule is for compatibility reasons,
+  it does not apply to other similar properties
+  like 'fill-color' or 'stroke-color'.
+
+  <div class="example">
+    For example,
+    given the following markup:
+    <pre class="lang-markup">
+      &lt;p>Highlight this &lt;em>and this&lt;/em>.&lt;/p>
+    </pre>
+    Any of the following rules
+    would suppress the default 'background-color'
+    for ''::selection''
+    in the <code>&lt;em></code> element
+    if given by the author:
+    <pre class="lang-css">
+      em::selection { color: initial; }
+      em::selection { color: inherit; }
+      em::selection { color: unset; }
+      em::selection { color: green; }
+      p::selection { color: green; }
+    </pre>
+  </div>
+
 <h3 id=highlight-bounds>
 Area of a Highlight</h3>
 
@@ -788,13 +841,21 @@ Cascading and Per-Element Highlight Styles</h3>
   for which that element is the <a>originating element</a>.
   When multiple styles conflict,
   the winning style is determined by the cascade.
+
   When any supported property is not given a value by the cascade,
-  its value is determined by inheritance from
+  or given a value of ''inherit'' or ''unset'',
+  its [=specified value=] is determined by inheritance from
   the corresponding <a>highlight pseudo-element</a>
   of its <a>originating element</a>'s parent element.
   This occurs regardless of whether that property is an <a>inherited property</a>,
   including [=registered custom properties=] where the [=inherits descriptor=] is false.
 
+  Additionally,
+  for [=highlight pseudo-elements=] originating from the root element,
+  the [=inherited value=] of 'color'
+  is ''currentColor'',
+  not the [=initial value=].
+
     <wpt>
     css/css-pseudo/active-selection-051.html
     css/css-pseudo/active-selection-052.html
@@ -857,17 +918,6 @@ Cascading and Per-Element Highlight Styles</h3>
     css/css-pseudo/cascade-highlight-002.html
     </wpt>
 
-
-  The UA must use its own highlight colors for ''::selection'' only
-  when neither 'color' nor 'background-color' has been specified by the author.
-
-  Note: This paired-cascading behavior
-  does not allow using the normal cascade
-  (e.g. '':root::selection'' rules in the <a>UA style sheet</a>)
-  to represent the OS default selection colors.
-  However it has been interoperably implemented in browsers
-  and is thus probably a Web-compatibility requirement.
-
 <h3 id="highlight-painting">
 Painting the Highlight</h3>