[css-backgrounds-3][css-backgrounds-4] Defined serialization of 'background-size' (#13053)

Author: SebastianZ

css-backgrounds-3/Overview.bs

@@ -1589,6 +1589,13 @@ Positioning Area: the 'background-origin' property</h3>
 	with other comma-separated background properties
 	to form each [=background image layer=].
 
+<h4 id="bg-size-serialization">
+Serialization of 'background-size' values</h4>
+
+	The [=specified value=] and [=computed value=] of the <<bg-size>> type
+	always serialize as two values, even when the second value is ''background-size/auto'',
+	due to the [[cssom#serializing-css-values|shortest, most backwards-compatible serialization principle]].
+
 <h3 id="background" oldids="the-background">
 Backgrounds Shorthand: the 'background' property</h3>
 
@@ -3743,6 +3750,8 @@ Changes since the 14 February 2023 Candidate Recommendation Snapshot</h3>
 			[=CSS/value definition syntax=].
 		<li>Fixed the definition for where box shadows apply.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/9286">Issue 9286</a>)
+		<li>Defined serialization of 'background-size' in [[#bg-size-serialization]].
+			(<a href="https://github.com/w3c/csswg-drafts/issues/7802">Issue 7802</a>)
 	</ul>
 
 

css-backgrounds-4/Overview.bs

@@ -1641,6 +1641,13 @@ Sizing Images: the 'background-size' property</h3>
 	with other comma-separated background properties
 	to form each [=background image layer=].
 
+<h4 id="bg-size-serialization">
+Serialization of 'background-size' values</h4>
+
+	The [=specified value=] and [=computed value=] of the <<bg-size>> type
+	always serialize as two values, even when the second value is ''background-size/auto'',
+	due to the [[cssom#serializing-css-values|shortest, most backwards-compatible serialization principle]].
+
 <h3 id='background-layers'>
 Background Image Layers: the 'background-tbd' shorthand property</h3>