[css-color-4] reference PNG sRGB chunk

Author: svgeesus

css-color-4/Overview.bs

@@ -358,8 +358,9 @@ Accessibility and Conveying Information By Color</h3>
 
 	Image formats may also use other, equivalent methods, often for brevity.
 
-	For example, PNG specifies a means to explicitly
-	tag an image as being in the sRGB colorspace,
+	For example, PNG specifies a means (the
+	<a href="https://www.w3.org/TR/PNG/#11sRGB">sRGB chunk</a>)
+	to explicitly tag an image as being in the sRGB colorspace,
 	without including the sRGB ICC profile.
 
 	Tagged RGB images,