[css-values-4] Define serialization of specified <position> #2274

css-values-4/Overview.bs

@@ -2669,8 +2669,8 @@ Combination of <<image>></h4>
 	This has been disallowed generically because it creates parsing ambiguities
 	when combined with other length or percentage components in a property value.
 
-	The canonical order when serializing is
-	the horizontal component followed by the vertical component.
+<h4 id="position-parsing">
+Parsing <<position>></h4>
 
 	When specified in a grammar alongside other keywords, <<length>>s, or <<percentage>>s,
 	<<position>> is <em>greedily</em> parsed;
@@ -2689,6 +2689,33 @@ Combination of <<image>></h4>
 		followed by a <<length>>.
 	</div>
 
+<h4 id="position-serialization">
+Serializing <<position>></h4>
+
+	When serializing the [=specified value=] of a <<position>>:
+
+	<dl class=switch>
+		<dt>If only one component is specified:
+		<dd>
+			* The implied ''center'' keyword is added,
+				and a 2-component value is serialized.
+
+		<dt>If two components are specified:
+		<dd>
+			* Keywords are serialized as keywords.
+			* <<length-percentage>>s are serialized as <<length-percentage>>s.
+			* Components are serialized horizontal first, then vertical.
+
+		<dt>If four components are specified:
+		<dd>
+			* Keywords and offsets are both serialized.
+			* Components are serialized horizontal first, then vertical.
+	</dl>
+
+	Note: [=Computed values=] are always serialized as two offsets (without keywords)
+	because the [=computed value=] does not preserve syntactic distinctions.
+
+
 <h4 id="combine-positions">
 Combination of <<position>></h4>