[css-values-4][css-backgrounds-3][editorial] Align grammar structure for <position> and <bg-position>

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -508,14 +508,15 @@ their initial position (after any <a href="#the-background-size">resizing</a>)
 within their corresponding <span>background positioning area.</span>
 
 <p>Where
-<pre class=prod><dfn><<bg-position>></dfn> =
+<pre class=prod><dfn><<bg-position>></dfn> = [
   [ left | center | right | top | bottom | <<length-percentage>> ]
 |
   [ left | center | right | <<length-percentage>> ]
   [ top | center | bottom | <<length-percentage>> ]
 |
   [ center | [ left | right ] <<length-percentage>>? ] &amp;&amp;
   [ center | [ top | bottom ] <<length-percentage>>? ]
+]
 </pre>
 
 <p>If only one value is specified, the second value is assumed to be

css-values-4/Overview.bs

@@ -2757,12 +2757,14 @@ Combination of <<image>></h4>
 
 	<pre class=prod>
 		<<position>> = [
-		  [ left | center | right ] || [ top | center | bottom ]
+		  [ left | center | right | top | bottom | <<length-percentage>> ]
+		|
+		  [ left | center | right ] && [ top | center | bottom ]
 		|
 		  [ left | center | right | <<length-percentage>> ]
-		  [ top | center | bottom | <<length-percentage>> ]?
+		  [ top | center | bottom | <<length-percentage>> ]
 		|
-		  [ [ left | right ] <<length-percentage>> ] &amp;&amp;
+		  [ [ left | right ] <<length-percentage>> ] &&
 		  [ [ top | bottom ] <<length-percentage>> ]
 		]
 	</pre>