diff --git a/css-backgrounds-3/Overview.bs b/css-backgrounds-3/Overview.bs
index ec1bb2221e8..ffea8cf1e5a 100644
--- a/css-backgrounds-3/Overview.bs
+++ b/css-backgrounds-3/Overview.bs
@@ -1589,6 +1589,13 @@ Positioning Area: the 'background-origin' property
with other comma-separated background properties
to form each [=background image layer=].
+
+Serialization of 'background-size' values
+
+ The [=specified value=] and [=computed value=] of the <> 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]].
+
Backgrounds Shorthand: the 'background' property
@@ -3743,6 +3750,8 @@ Changes since the 14 February 2023 Candidate Recommendation Snapshot
[=CSS/value definition syntax=].
Fixed the definition for where box shadows apply.
(Issue 9286)
+ Defined serialization of 'background-size' in [[#bg-size-serialization]].
+ (Issue 7802)
diff --git a/css-backgrounds-4/Overview.bs b/css-backgrounds-4/Overview.bs
index 35e125c3016..b315678f5da 100644
--- a/css-backgrounds-4/Overview.bs
+++ b/css-backgrounds-4/Overview.bs
@@ -1641,6 +1641,13 @@ Sizing Images: the 'background-size' property
with other comma-separated background properties
to form each [=background image layer=].
+
+Serialization of 'background-size' values
+
+ The [=specified value=] and [=computed value=] of the <> 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]].
+
Background Image Layers: the 'background-tbd' shorthand property