[css-color-4] When interpolating, always convert colorspace, so that …

…powerless components become missing #8609

css-color-4/Overview.bs

@@ -4480,8 +4480,11 @@ Color Interpolation</h2>
 
 	<ol>
 		<li>checking the two colors for [=analogous components=] which will be carried forward</li>
-		<li>(if required) converting them to a given color space
-			which will be referred to as the <dfn export>interpolation color space</dfn> below</li>
+		<li>converting them to a given color space
+			which will be referred to as the <dfn export>interpolation color space</dfn> below. 
+			If one or both colors are already in the interpolation colorspace,
+			this conversion changes any [=powerless=] components to [=missing=] values
+		</li>
 		<li>(if required) re-inserting carried-forward values in the converted colors</li>
 		<li>(if required) fixing up the hues, depending on the selected <<hue-interpolation-method>></li>
 		<li>changing the color components to premultiplied form</li>