[css-color-5] Add gradient rendering w3c#7771

Author: svgeesus

css-color-5/Overview.bs

@@ -991,6 +991,11 @@ which are then carried forward as missing.
 	If an implementation failed to do this carrying forward,
 	the grayish --darker-bg would have a hue of 0,
 	giving an undesirable reddish tint at the start of the  gradient.
+
+	<figure id="fig-gradient-missing">
+		<img src="./images/rectangular-missing.png" alt="">
+		<figcaption>Correct (above) and incorrect (below, reddish) gradients.</figcaption>
+	</figure>
 </div>
 
 <h3 id="relative-RGB">Relative sRGB Colors</h3>