Add WG resolution about unitless 0 to Images 4 (related to #1162)

Author: LeaVerou

css-images-4/Overview.bs

@@ -954,6 +954,7 @@ Conic Gradients: the ''conic-gradient()'' notation</h3>
 		<dd>
 			The entire gradient is rotated by this angle.
 			If omitted, defaults to ''0deg''.
+			The unit identifier may be omitted if the <<angle>> is zero.
 
 		<dt><dfn><<position>></dfn>
 		<dd>
@@ -1201,6 +1202,7 @@ Gradient Color-Stops</h3>
 	While every color stop conceptually has at least one position,
 	the position can be omitted in the syntax.
 	(It gets automatically filled in by the user agent; see below for details.)
+	The unit identifier may be omitted if the position is zero.
 
 	Between two <a>color stops</a> there can be a <dfn local-lt="color hint">color interpolation hint</dfn>,
 	which specifies how the colors of the two <a>color stops</a> on either side