[css-images] Explicitly allow <zero> for gradient rotation

Author: cdoublev

css-images-3/Overview.bs

@@ -334,7 +334,7 @@ Linear Gradients: the ''linear-gradient()'' notation {#linear-gradients}
 	<pre class=prod>
 		<<linear-gradient()>> = linear-gradient( [ <<linear-gradient-syntax>> ] )
 
-		<dfn>&lt;linear-gradient-syntax></dfn> = [ <<angle>> | to <<side-or-corner>> ]? , <<color-stop-list>>
+		<dfn>&lt;linear-gradient-syntax></dfn> = [ <<angle>> | <<zero>> | to <<side-or-corner>> ]? , <<color-stop-list>>
 
 		<dfn>&lt;side-or-corner></dfn> = [left | right] || [top | bottom]
 	</pre>

css-images-4/Overview.bs

@@ -1271,7 +1271,7 @@ Linear Gradients: the ''linear-gradient()'' notation {#linear-gradients}
 
 	<pre class=prod>
 		<dfn>&lt;linear-gradient-syntax></dfn> =
-			[ [ <<angle>> | to <<side-or-corner>> ] || <<color-interpolation-method>> ]? ,
+			[ [ <<angle>> | <<zero>> | to <<side-or-corner>> ] || <<color-interpolation-method>> ]? ,
 			<<color-stop-list>>
 		<dfn>&lt;side-or-corner></dfn> = [left | right] || [top | bottom]
 	</pre>
@@ -1560,14 +1560,14 @@ Conic Gradients: the ''conic-gradient()'' notation</h3>
 	<pre class='prod'>
 		<dfn>conic-gradient()</dfn> = conic-gradient( [ <<conic-gradient-syntax>> ] )
 		<dfn>&lt;conic-gradient-syntax></dfn> =
-			[ [ [ from <<angle>> ]? [ at <<position>> ]? ] || <<color-interpolation-method>> ]? ,
+			[ [ [ from [ <<angle>> | <<zero>> ] ]? [ at <<position>> ]? ] || <<color-interpolation-method>> ]? ,
 			<<angular-color-stop-list>>
 	</pre>
 
 	The arguments are defined as follows:
 
 	<dl dfn-type=value dfn-for="conic-gradient(), repeating-conic-gradient()">
-		<dt><dfn><<angle>></dfn>
+		<dt><dfn><<angle>> | <<zero>></dfn>
 		<dd>
 			The entire gradient is rotated by this angle.
 			If omitted, defaults to ''0deg''.