[mediaqueries-5] Remove inverted-colors UA rule (#9812)

Change it to an author suggestion. Closes #9674.

mediaqueries-5/Overview.bs

@@ -1985,8 +1985,7 @@ Detecting inverted colors on the display: the 'inverted-colors' feature</h3>
 
 			This value must not match
 			if the user agent has done some kind of content aware inversion
-			such as one that preserves the images
-			(except through its UA style sheet, see below).
+			such as one that preserves the images.
 
 			Note: This is because the goal of this media feature
 			is to enable authors to mitigate the undesirable effects of the non content aware approach
@@ -1996,21 +1995,22 @@ Detecting inverted colors on the display: the 'inverted-colors' feature</h3>
 
 	</dl>
 
-	User agents must add the following rule to their [=UA style sheet=]:
+	<div class="example">
+		Depending on their style sheet,
+		authors may wish to invert images and videos:
 
-	<pre><code class=lang-css>
-		@media (inverted-colors) {
-			img:not(picture>img), picture, video { filter: invert(100%); }
-		}
-	</code></pre>
+		<pre class="lang-css">
+			@media (inverted-colors) {
+				img:not(picture > img), picture, video {
+					filter: invert(100%);
+				}
+			}
+		</pre>
 
-	<div class="example">
-		In addition to this UA style sheet rule,
-		and depending on their style sheet,
-		authors may also wish to invert images injected via CSS (such as backgrounds),
-		or to disable shadows:
+		Authors may also invert images injected via CSS (such as backgrounds),
+		or disable shadows:
 
-		<pre>
+		<pre class="lang-css">
 		@media (inverted-colors) {
 			* {
 				text-shadow: none !important;