[css-backgrounds-4] Incorporate logical keywords to background-position

#9690

css-backgrounds-4/Overview.bs

@@ -205,49 +205,48 @@ Tiling Images Shorthand: the 'background-repeat' property</h3>
 <h3 id="the-background-position">
 Background Positioning: the 'background-position' shorthand property</h3>
 
-	<pre class="propdef shorthand">
-		Name: background-position
-		Value: <<bg-position>>#
+	<pre class="propdef">
+	Name: background-position
+	Value: <<bg-position>>#
+	Initial: 0% 0%
+	Applies to: all elements
+	Inherited: no
+	Percentages: refer to size of <span class=index>background positioning area</span>
+	    <em>minus</em> size of background image; see text
+	Computed value: a list,
+	    each item a pair of offsets (horizontal and vertical) from the top left origin,
+	    each offset given as a computed <<length-percentage>> value
+	Animation type: repeatable list
 	</pre>
 
-	See [[CSS3BG]] for definition.
-
-	<p>Where
-	<pre class=prod><dfn>&lt;bg-position&gt;</dfn> = [
-	  [ left | center | right | top | bottom | start | end | <<length-percentage>> ]
-	|
-	  [ left | center | right | x-start | x-end | <<length-percentage>> ]
-	  [ top | center | bottom | y-start | y-end | <<length-percentage>> ]
-	|
-	  [ center | [ left | right | x-start | x-end ] <<length-percentage>>? ] &amp;&amp;
-	  [ center | [ top | bottom | y-start | y-end ] <<length-percentage>>? ]
-	|
-	  [ center | [ start | end ] <<length-percentage>>? ]
-	  [ center | [ start | end ] <<length-percentage>>? ]	]</pre>
-
-	Values have the following meanings:
-	<dl>
-		<dt>One value
-		<dd>
-			If only one value is given,
-			and that value is ''background-position/start'' or ''background-position/end'',
-			then the keyword is duplicated;
-			otherwise the second keyword defaults to ''background-position/center''.
-			The resulting value is treated as a two-component value.
+	If [=background images=] have been specified,
+	this property specifies their initial position
+	(after any <a href="#background-size">resizing</a>)
+	within their corresponding [=background positioning area=].
 
-		<dt>More than one value
-		<dd>
-			If the value contains a ''background-position/start'' or ''background-position/end'' keyword,
-			then the shorthand sets
-			'background-position-inline' and 'background-position-block' to the specified values.
-			Otherwise
-			the shorthand sets
-			'background-position-x' and 'background-position-y' to the specified values.
-
-			Issue: Specify the value assignment in more detail. Should expand just like Level 3.
-	</dl>
+	This property is a [=shorthand property=] that sets
+	'background-position-x', 'background-position-y', 'background-position-block', and 'background-position-inine'
+	in a single declaration.
+
+	Its value is given as a comma-separated list
+	of <dfn><<bg-position>></dfn> values, which are interpreted as <<position>> values
+	with the resized [=background image=] as the [=alignment subject=]
+	and the [=background positioning area=] as the [=alignment container=].
+
+	<pre class=prod>
+		<<bg-position>> =  <<position>> | <<position-three>>
+		<position-three> = [
+		  [ left | center | right ] && [ [ top | bottom ] <<length-percentage>> ]
+		|
+		  [ [ left | right ] <<length-percentage>> ] && [ top | center | bottom ]
+		]
+	</pre>
+
+	The omitted <<length-percentage>> in the 'background-position'-specific
+	<<position-three>> syntax variant
+	defaults to ''0%''.
 
-	Issue: Specify what happens to set of properties that are not set. Maybe they're just not set?
+	Issue(9690): Specify how the [=longhand properties=] are set.
 
 <h4 id="background-position-longhands">
 Background Positioning Longhands: the 'background-position-x', 'background-position-y', 'background-position-inline', and 'background-position-block' properties</h4>