[css-pseudo] text decorations on highlight pseudos (closes #6022) (#6666)

Clarify highlight decoration semantics

Author: delan

css-pseudo-4/Overview.bs

@@ -917,15 +917,22 @@ Shadows</h4>
 Text and Text Decorations</h4>
 
   A <a>highlight pseudo-element</a>
-  suppresses the normal drawing of any associated text
-  (and the text decorations that had been applied to that text).
+  suppresses the normal drawing of any associated text,
+  and the text decorations
+  (other than shadows)
+  that had been applied to that text.
   Instead the topmost active <a>highlight overlay</a>
   redraws that text
   (and those decorations)
   over all the <a>highlight overlay</a> backgrounds
   <!-- (and outlines, if any) -->
   using that highlight’s own 'color'.
 
+  Note: This means that unlike shadows,
+  line decorations and emphasis marks
+  won’t be obscured by any highlight overlay backgrounds
+  that are drawn for the associated text.
+
   For this purpose,
   ''currentColor'' on a [=highlight pseudo-element=]’s 'color' property represents
   the 'color' of the next <a>highlight pseudo-element</a> layer below,
@@ -959,7 +966,7 @@ Text and Text Decorations</h4>
   <div class="example">
     For example, assuming the original text has an underline and a strike-through,
     that ''::selection'' applies an underline
-    and ''::target-text'' appplies both overline and strike-through,
+    and ''::target-text'' applies both overline and strike-through,
     the following are both conformant painting orders:
     <div style="columns: 20em">
       <ol style="break-inside: avoid; margin-top: 0;">