[css-backgrounds-4] Clarified examples for background layers property

Author: SebastianZ

css-backgrounds-4/Overview.bs

@@ -210,29 +210,33 @@ Background Image Layers: the 'background-tbd' shorthand property</h3>
 		<a href="https://github.com/w3c/csswg-drafts/issues/9083">issue 9083</a>.
 
 	<div class="example" id="background-layer-example">
-		This example sets three background layers,
-		the first one including the image, position and repetition,
-		the second including the image, position, and size,
-		and the third one only consisting of the image.
+		This example sets two background layers later in the cascade.
+		By using 'background-tbd', the previously set 'background-color'
+		won't be overridden.
 
 		<pre class="lang-css">
+		p {
+			background-color: green;
+		}
+
 		p {
 			background-tbd:
-				url(a.png) top left no-repeat,
-				url(b.png) center / 100% 100% no-repeat,
-				url(c.png);
+				url(a.png) top left,
+				url(b.png) top left no-repeat;
 		}
 		</pre>
 	</div>
 
 	<div class="invalid example" id="invalid-background-layer-example">
 		This example tries to set the background color in addition to
-		the background image. For that to work, 'background' needs
-		to be used instead of 'background-tbd'.
+		the background image. But for that to work,
+		'background' needs to be used instead of 'background-tbd'.
+		So the 'background-tbd' declaration will be dropped.
 
 		<pre class="lang-css">
 		p {
-			background-tbd: url(c.png) white;
+			background: url(pass.png) green;   /* valid */
+			background-tbd: url(fail.png) red; /* invalid */
 		}
 		</pre>
 	</div>