[css-backgrounds-3] Define background-position serialization. w3c#2274

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -657,6 +657,17 @@ other comma-separated background properties to form each background
 image layer.
 
 
+<h4 id="bg-position-serialization">
+Serialization of 'background-position' values</h4>
+
+The [=specified value=] and [=computed value=] of the <<bg-position>> type
+serializes exactly as defined in [[CSS-VALUES-4]] for <<position>>.
+For 3-value productions
+(which are not valid in <<position>>),
+the [=specified value=] serialization
+is identical to the equivalent 4-value syntax
+except that the omitted offset remains omitted.
+
 
 <h3 id="background-clip" oldids="the-background-clip">Painting Area: the 'background-clip' property</h3>
 
@@ -2561,6 +2572,8 @@ Changes</h2>
 Changes since the 14 February 2024 Candidate Recommendation Snapshot</h3>
 
 <ul>
+  <li>Defined serialization of 'background-position' in [[#bg-position-serialization]].
+    (<a href="https://github.com/w3c/csswg-drafts/issues/2274">Issue 2274</a>)
   <li>Reverted <a href="#bg-color-move">change moving &lt;color> to the front of &lt;final-bg-layer></a>
     in the 'background' shorthand.
     (<a href="https://github.com/w3c/csswg-drafts/issues/8496">Issue 8496</a>)