[css-pseudo-4] Rewrite ::selection et al. cascade using inheritance mechanism and make everything fit together into some kind of coherent model. w3c#2474

Author: fantasai

css-pseudo-4/Overview.bs

@@ -491,28 +491,14 @@ Styling Highlights</h3>
 
   Issue: Are there any other properties that should be included here?
 
-  Note: Historically (and at the time of writing)
-  only 'color' and 'background-color' have been interoperably supported.
-
-  If 'color' is not specified, the text (and text decoration)'s
-  unselected color must be used for the highlighted text.
-  (As usual, the initial 'background-color' is ''transparent''.)
-
-  Issue: Can we reuse ''currentColor'' for this, now that it computes to itself?
-
-  The UA should use the OS-default highlight colors
-  when neither 'color' nor 'background-color' has been specified by the author.
-
-  Note: This paired-cascading behavior
-  does not allow <a href="http://lists.w3.org/Archives/Public/www-style/2008Oct/0268.html">using the normal cascade</a>
-  to represent the OS default selection colors.
-  However it has been interoperably implemented in browsers
-  and is thus probably a Web-compatibility requirement.
-
   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.
+  applied to the text
+  by the <a>originating element</a> and its ancestors and descendants.
+
+  Note: Historically (and at the time of writing)
+  only 'color' and 'background-color' have been interoperably supported.
 
 <h3 id=highlight-bounds>
 Area of a Highlight</h3>
@@ -552,12 +538,17 @@ Area of a Highlight</h3>
 <h3 id=highlight-cascade>
 Cascading and Per-Element Highlight Styles</h3>
 
-  Each element draws its own portion of the <a>highlight overlay</a>,
-  which receives the styles specified by the <a>highlight pseudo-element</a> styles
-  for which that element or one of its ancestors is the <a>originating element</a>.
+  Each element draws its own active portions of the <a>highlight overlays</a>,
+  which receives the styles specified by
+  the corresponding <a>highlight pseudo-element</a> styles
+  for which that element is the <a>originating element</a>.
   When multiple styles conflict,
-  the winning style is the one belonging to the innermost element
-  after cascading.
+  the winning style is determined by the cascade.
+  When any supported property is not given a value by the cascade,
+  it's value is determined by inheritance from
+  the corresponding <a>highlight pseudo-element</a>
+  of its <a>originating element</a>'s parent element
+  (regardless of whether that property is an <a>inherited property</a>).
 
   <div class="example">
     For example, if the following rules were applied:
@@ -570,17 +561,11 @@ Cascading and Per-Element Highlight Styles</h3>
     <pre>
       &lt;p>Highlight this &lt;em>and this&lt;/em>.&lt;/p>
     </pre>
-    The highlight would be green throughout,
+    The selection highlight would be green throughout,
     with yellow text outside the <code>&lt;em></code> element
     and orange text inside it.
   </div>
 
-  Issue: This could alternately be described in terms of inheritance.
-  The observable differences would be in how ''inherit'' and ''unset'' behave.
-  Should it inherit from the parent ''::selection''
-  or the <a>originating element</a>?
-  Opera does the former, Gecko/Blink the latter.
-
   <p class="advisement">
   Authors wanting multiple selections styles should use
   <strong>'':root::selection''</strong>
@@ -611,35 +596,57 @@ Cascading and Per-Element Highlight Styles</h3>
     </pre>
   </div>
 
+  The UA should use the OS-default highlight colors for ''::selection''
+  when neither 'color' nor 'background-color' has been specified by the author.
+
+  Note: This paired-cascading behavior
+  does not allow using the normal cascade
+  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>
 
   Each <a>highlight pseudo-element</a> draws its background
-  over the active portion of the overlay,
+  over the corresponding portion of the <a>highlight overlay</a>,
   painting it
   immediately below any positioned descendants
   (i.e. just before step 8 in <a href="https://www.w3.org/TR/CSS21/zindex.html">CSS2.1&sect;E.2</a>).
-  It also suppresses the drawing of any selected text
-  (and any text decorations applied to that text)
-  and instead redraws that text
+  Its outline, if any, is painted immediately over its background.
+  The ''::selection'' overlay is drawn
+  over the ''::spelling-error'' overlay
+  which is drawn over the ''::grammar-error'' overlay.
+
+  A <a>highlight pseudo-element</a>
+  also suppresses the drawing of any selected text
+  (and any text decorations applied to that text).
+  Instead the topmost active <a>highlight overlay</a>
+  redraws that text
   (and its decorations)
-  over the <a>highlight overlay</a> background
-  using the specified 'color'.
+  over the <a>highlight overlay</a> backgrounds
+  (and outlines, if any)
+  using its own 'color',
+  with ''currentColor'' representing the 'color'
+  of the next <a>highlight pseudo-element</a> layer below,
+  falling back finally to that of the <a>originating element</a>
+  (the colors that would otherwise be used).
+  Any text decorations introduced by each <a>highlight pseudo-element</a>
+  are stacked in the same order as their backgrounds
+  over the text’s original decorations
+  and are all drawn, in their own colors.
 
   Issue: What should happen with text shadows?
   Drawing them in their original color is disconcerting if that color is not a shade of gray.
+  Maybe if the overlay has a background, suppress any text shadows below it?
 
   For non-replaced content, the UA must honor the 'color' and 'background-color'
   (including their alpha channels) as specified.
   However, for replaced content, the UA should create a semi-transparent wash
   to coat the content so that it can show through the selection.
   This wash should be of the specified 'background-color' if that is not ''transparent'',
   else of the specified 'color';
-  however the UA may adjust the alpha channel if it is opaque.
-
-  The ''::selection'' overlay is drawn
-  over the ''::spelling-error'' overlay
-  which is drawn over the ''::grammar-error'' overlay.
+  however the UA may adjust the alpha channel.
 
 <h3 id="highlight-security">
 Security and Privacy Considerations</h3>