[css-color-4] Add example of carried forward alpha for premultiplication, fix w3c#7536

Author: svgeesus

css-color-4/Overview.bs

@@ -4536,14 +4536,34 @@ Interpolating with Missing Components</h3>
 		</pre>
 	</div>
 
+	If the carried forward [=missing component=]
+	is alpha, the color must be premultiplied with this carried forward value,
+	not with the zero value that would have resulted from color conversion.
+
+	<div class="example" id="ex-oklch-missing-alpha">
+		For example, if these two colors are interpolated,
+		the second of which has a missing alpha:
+
+		<pre>
+			<span class="swatch" style="--color: rgb(86.6% 62.7% 86.7% / 0.5)"></span> oklch(0.783 0.108 326.5 / 0.5)
+			<span class="swatch" style="--color: rgb(48.4% 0% 22.8% / 0)"></span> oklch(0.392 0.4 0 / none)
+		</pre>
+
+		Then the actual colors to be interpolated are
+
+		<pre>
+			<span class="swatch" style="--color: rgb(86.6% 62.7% 86.7% / 0.5)"></span> oklch(78.3% 0.108 326.5 / 0.5)
+			<span class="swatch" style="--color: rgb(48.4% 0% 22.8% / 0.5)"></span> oklch(39.2% 0.4 0 / 0.5)
+		</pre>
+
+		giving the premultiplied Oklch values [0.3915, 0.054, 326] and [0.196, 0.2, 0]. 
+	</div>
+
 	If both colors are [=missing=]
 	a given component,
 	the interpolated color
 	will also be [=missing=] that component.
 
-	<wpt>
-		
-	</wpt>
 
 <h3 id="interpolation-alpha">
 Interpolating with Alpha</h3>