Check in (slightly less verbose version of) Tab's less-confusing grammar for <position>. <http://lists.w3.org/Archives/Public/www-style/2011Dec/0447.html>

Author: fantasai

css3-background/Overview.html

@@ -1024,10 +1024,13 @@ <h3 id=the-background-position><span class=secno>3.6. </span>The &lsquo;<a
   <p>Where
 
   <pre class=prod><dfn id=ltpositiongt><var>&lt;position&gt;</var></dfn> = [
-  [ top | bottom ]
+ [ left | center | right | top | bottom | <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]
 |
-  [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> | left | center | right ]
-  [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> | top | center | bottom ]?
+ [ left | center | right ] &&
+ [ top | center | bottom ]
+|
+  [ left | center | right | <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]
+  [ top | center | bottom | <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]
 |
   [ center | [ left | right ] [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ] &amp;&amp;
   [ center | [ top | bottom ] [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]

css3-background/Overview.src.html

@@ -631,10 +631,13 @@ <h3 id="the-background-position">The 'background-position' property</h3>
 
 <p>Where
 <pre class=prod><dfn><var>&lt;position&gt;</var></dfn> = [
-  [ top | bottom ]
+ [ left | center | right | top | bottom | <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]
 |
-  [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> | left | center | right ]
-  [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> | top | center | bottom ]?
+ [ left | center | right ] &&
+ [ top | center | bottom ]
+|
+  [ left | center | right | <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]
+  [ top | center | bottom | <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]
 |
   [ center | [ left | right ] [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ] &amp;&amp;
   [ center | [ top | bottom ] [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]