[css-images-4] Allow <zero> for angular color stop/hint

Author: cdoublev

css-images-4/Overview.bs

@@ -1843,8 +1843,8 @@ Color Stop Lists</h4>
 		<dfn>&lt;angular-color-stop-list></dfn> =
 			<<angular-color-stop>> , [ <<angular-color-hint>>? , <<angular-color-stop>> ]#?
 		<dfn>&lt;angular-color-stop></dfn> = <<color>> <<color-stop-angle>>?
-		<dfn>&lt;angular-color-hint></dfn> = <<angle-percentage>>
-		<dfn>&lt;color-stop-angle></dfn> = <<angle-percentage>>{1,2}
+		<dfn>&lt;angular-color-hint></dfn> = <<angle-percentage>> | <<zero>>
+		<dfn>&lt;color-stop-angle></dfn> = [ <<angle-percentage>> | <<zero>> ]{1,2}
 
 		<dfn>&lt;color-stop></dfn> = <<color-stop-length>> | <<color-stop-angle>>
 	</pre>