[css-backgrounds-4] 'border-area' value in 'background' shorthand sets 'background-origin' to 'border-box' (#13042)

Author: SebastianZ

css-backgrounds-4/Overview.bs

@@ -1741,6 +1741,9 @@ Backgrounds Shorthand: the 'background' property</h3>
 
 	If one <<visual-box>> value is present
 	then it sets both 'background-origin' and 'background-clip' to that value.
+	However, there is one exception:
+	if the value is ''background-clip/border-area'',
+	it is set for 'background-clip', while 'background-origin' is set to ''background-origin/border-box''.
 	If two values are present,
 	then the first sets 'background-origin' and the second 'background-clip'.