[css-backgrounds-4] Handle omitted origin keywords in background-position-*

Author: fantasai

css-backgrounds-4/Overview.bs

@@ -88,7 +88,7 @@ Background Positioning Longhands: the 'background-position-x', 'background-posit
 
 	<pre class='propdef'>
 		Name: background-position-x
-		Value: [ center | [ left | right | x-start | x-end ] [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]#
+		Value: [ center | [ left | right | x-start | x-end ]? [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]#
 		Initial: left
 		Applies to: all
 		Inherited: no
@@ -98,10 +98,11 @@ Background Positioning Longhands: the 'background-position-x', 'background-posit
 	</pre>
 
 	This property specifies the background position's horizontal component.
+	An omitted origin keyword is assumed to be ''left''.
 
 	<pre class='propdef'>
 		Name: background-position-y
-		Value: [ center | [ top | bottom | y-start | y-end ] [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]#
+		Value: [ center | [ top | bottom | y-start | y-end ]? [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]#
 		Initial: left
 		Applies to: all
 		Inherited: no
@@ -111,10 +112,11 @@ Background Positioning Longhands: the 'background-position-x', 'background-posit
 	</pre>
 
 	This property specifies the background position's vertical component.
+	An omitted origin keyword is assumed to be ''top''.
 
 	<pre class='propdef'>
 		Name: background-position-inline
-		Value: [ center | [ start | end ] [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]#
+		Value: [ center | [ start | end ]? [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]#
 		Initial: not applicable (initial value comes from physical property)
 		Applies to: all
 		Inherited: no
@@ -124,10 +126,11 @@ Background Positioning Longhands: the 'background-position-x', 'background-posit
 	</pre>
 
 	This property specifies the background position's inline-axis component.
+	An omitted origin keyword is assumed to be ''start''.
 
 	<pre class='propdef'>
 		Name: background-position-block
-		Value: [ center | [ start | end ] [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]#
+		Value: [ center | [ start | end ]? [ <var>&lt;percentage&gt;</var> | <var>&lt;length&gt;</var> ]? ]#
 		Initial: not applicable (initial value comes from physical property)
 		Applies to: all
 		Inherited: no
@@ -137,6 +140,7 @@ Background Positioning Longhands: the 'background-position-x', 'background-posit
 	</pre>
 
 	This property specifies the background position's block-axis component.
+	An omitted origin keyword is assumed to be ''start''.
 
 <h2 id="corners">
 Corners</h2>