[css-color-4] Add mention of PNG cICP chunk for tagging images

Author: svgeesus

css-color-4/Overview.bs

@@ -720,7 +720,10 @@ Transparency: the 'opacity' property</h3>
 	without including the sRGB ICC profile.
 
 	Similarly, PNG specifies a compact means
-	(the <a href=""></a> )
+	(the <a href="https://www.w3.org/TR/png-3/#cICP-chunk">cICP chunk</a>)
+	to explicitly tag an image as being one of various SDR or HDR color spaces,
+	such as Display P3 or BT.2100 HLG,
+	without including an ICC profile.
 
 	Tagged RGB images,
 	and tagged images using a transformation of RGB such as YCbCr,