[css-images-4] Allow a single color stop with two positions

Author: cdoublev

css-images-4/Overview.bs

@@ -1833,16 +1833,20 @@ Color Stop Lists</h4>
 
 	<pre class=prod>
 		<dfn>&lt;color-stop-list></dfn> =
-			<<linear-color-stop>> , [ <<linear-color-hint>>? , <<linear-color-stop>> ]#
+			  <<linear-color-stop>> , [ <<linear-color-hint>>? , <<linear-color-stop>> ]#
+			| <<single-linear-color-stop>>
 		<dfn>&lt;linear-color-stop></dfn> = <<color>> <<color-stop-length>>?
 		<dfn>&lt;linear-color-hint></dfn> = <<length-percentage>>
 		<dfn>&lt;color-stop-length></dfn> = <<length-percentage>>{1,2}
+		<dfn>&lt;single-linear-color-stop></dfn> = <<color>> <<length-percentage>>{2}
 
 		<dfn>&lt;angular-color-stop-list></dfn> =
-			<<angular-color-stop>> , [ <<angular-color-hint>>? , <<angular-color-stop>> ]#
+			  <<angular-color-stop>> , [ <<angular-color-hint>>? , <<angular-color-stop>> ]#
+			| <<single-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;single-angular-color-stop></dfn> = <<color>> <<angle-percentage>>{2}
 
 		<dfn>&lt;color-stop></dfn> = <<color-stop-length>> | <<color-stop-angle>>
 	</pre>