[css-color-5][editorial] more alpha examples

Author: svgeesus

css-color-5/Overview.bs

@@ -1199,6 +1199,18 @@ in the range [0, 360].
 	drawing their values from the [=origin color=],
 	but the opacity is set to ''80%'' to make it slightly transparent,
 	regardless of what the [=origin color's=] opacity was.
+
+	While this example is syntactically correct,
+	and illustrates what happens,
+	there is a simpler form for this specific case of alpha-only manipulation,
+	which avoids having to list the unchanged color components:
+
+	<pre highlight=css>
+	html { --bg-color: <span class="swatch" style="--color: blue"></span> blue; }
+	.overlay {
+		background: <span class="swatch" style="--color: rgb(0,0,255,80%)"></span> alpha(from var(--bg-color) / 80%);
+	}
+	</pre>
 </div>
 
 <div class="example">