[css-pseudo-4] Set text-shadow: none on root ::selection. Fix ::spelling-error/::grammar-error rules to also use :root::pseudo pattern for proper overrideability. #3605

Author: fantasai

css-pseudo-4/Overview.bs

@@ -464,8 +464,11 @@ Selecting Highlighted Content: the ''::selection'',  ''::inactive-selection'', '
 
     The following addition is made to the default UA stylesheet:
 	<pre><code class="lang-css">
-		::spelling-error { text-decoration-line: spelling-error; }
-		::grammar-error { text-decoration-line: grammar-error; }
+		/* 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; }
+		/* Ensure contrast on text with shadows */
+		:root::selection      { text-shadow: none; }
 	</code></pre>
 
   Note: A future level of CSS may introduce ways to create