[css-values-4] Add genericized definition for coordinating list properties. #7164

fantasai · fantasai · commit dce5c9df4fdc · 2022-10-10T17:00:38.000-04:00
diff --git a/css-values-4/Overview.bs b/css-values-4/Overview.bs
@@ -4725,9 +4725,51 @@ Appendix A: Recommended Minimum Ranges and Precision of Computed Values</h2>
 		</span>
 -->
 
+<h2 id="linked-properties">
+Appendix A: Coordinating List-Valued Properties</h2>
+
+	Some list-valued properties have coordinated effects:
+	each item in their value list applies to a distinct effect,
+	and corresponding entries in each property's list
+	all refer to the same effect.
+	Often the coordinating values can also be specified together
+	as a single entry in a list-valued [=shorthand property=].
+
+	A typical example is the list-valued 'background-*' properties,
+	which can specify <a href="https://www.w3.org/TR/css-backgrounds-3/#layering">multiple background image layers</a>.
+	For each property controlling how the image is sized, tiled, placed, etc.,
+	the <var>N</var>th item in its list describes some effect that applies
+	to the <var>N</var>th background image.
+
+	A <dfn export lt="coordinating list property | coordinating list property group">coordinating list property group</dfn> creates
+	a <dfn export>coordinated value list</dfn>,
+	which has, for each entry,
+	a value from each property in the group;
+	these are used together to define a single effect,
+	such as a background image layer
+	or an animation.
+	The [=coordinated value list=] is assembled as follows:
+
+	* The length of the [=coordinated value list=] is determined by
+		the number of items specified in one particular [=coordinating list property=],
+		the <dfn export>coordinating list base property</dfn>.
+		(In the case of backgrounds, this is the 'background-image' property.)
+
+	* The <var>N</var>th value of the [=coordinated value list=] is constructed
+		by collecting the <var>N</var>th [=use value=] of each [=coordinating list property=]
+
+	* If a [=coordinating list property=] has too many values specified,
+		excess values at the end of its list are not [=used value|used=].
+
+	* If a [=coordinating list property=] has too few values specified,
+		its value list is repeated to add more [=used values=].
+
+	* The [=computed values=] of the [=coordinating list properties=]
+		are not affected by such truncation or repetition.
+
 
 <h2 id='iana'>
-Appendix A: IANA Considerations</h2>
+Appendix B: IANA Considerations</h2>
 
 <h3 id='about-invalid'>
 Registration for the <code>about:invalid</code> URL scheme</h3>
@@ -4756,7 +4798,7 @@ Registration for the <code>about:invalid</code> URL scheme</h3>
 	</table>
 
 <h2 id="deprecated-quirky-length">
-Appendix B: Quirky Lengths</h2>
+Appendix C: Quirky Lengths</h2>
 
 	When CSS is being parsed in [=quirks mode=],
 	<dfn type><<quirky-length>></dfn> is a type of <<length>>
@@ -4965,6 +5007,8 @@ Additions Since Level 3</h3>
 		<li>A non-integer in a calc() automatically rounds to the nearest integer
 			when used where an <<integer>> is required.
 		<li>Defined [[#calc-serialize|serialization]] of [=math functions=].
+		<li>Added a genericized definition of [=coordinating list property groups=],
+			to make it easier to reference the coordinating behavior of the 'background' properties.
 	</ul>
 
 <h2 class="no-num" id="sec-pri">
