[css-images-4] Allow just one color stop. w3c#10092

Author: tabatkins

css-images-4/Overview.bs

@@ -1830,18 +1830,18 @@ Color Stop Lists</h4>
 
 	<a>Color stops</a> and [=transition hints=] are specified
 	in a <dfn export>color stop list</dfn>,
-	which is a list of two or more [=color stops=]
+	which is a list of one or more [=color stops=]
 	interleaved with optional [=transition hints=]:
 
 	<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>> ]#?
 		<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;angular-color-stop-list></dfn> =
-			<<angular-color-stop>> , [ <<angular-color-hint>>? , <<angular-color-stop>> ]#
+			<<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}