[css-values-5] Add sibling-count/index(). w3c#4559

Author: tabatkins

css-values-5/Overview.bs

@@ -17,6 +17,7 @@ Default Highlight: css
 </pre>
 <pre class='link-defaults'>
 spec: css-values-4; type: dfn; text: determine the type of a calculation
+spec: selectors-4; type: dfn; text: selector
 </pre>
 <style>
 code, small { white-space: nowrap }
@@ -1403,6 +1404,37 @@ Generating/Caching Random Values: the <<random-caching-options>> value</h4>
 
 
 
+<h2 id="tree-counting">
+Tree Counting Functions: the ''sibling-count()'' and ''sibling-index()'' notations</h2>
+
+	The <dfn>sibling-count()</dfn> [=functional notation=] represents,
+	as an <<integer>>,
+	the total number of child <l spec=css-display-3>[=elements=]</l>
+	in the parent of the element on which the notation is used.
+
+	The <dfn>sibling-index()</dfn> [=functional notation=] represents,
+	as an <<integer>>,
+	the index of the element
+	on which the notation is used
+	among the children of its parent.
+	Like '':nth-child()'',
+	''sibling-index()'' is 1-indexed.
+
+	Note: The ''counter()'' function can provide similar abilities as ''sibling-index()'',
+	but returns a <<string>> rather than an <<integer>>.
+
+	When used on a [=pseudo-element=],
+	these both resolve as if specified
+	on its [=ultimate originating element=].
+
+	Note: Like the rest of CSS (other than [=selectors=]),
+	''sibling-count()'' and ''sibling-index()''
+	operate on the [=flat tree=].
+
+	Note: These functions may, in the future,
+	be extended to accept an ''of <<selector>>'' argument,
+	similar to '':nth-child()'',
+	to filter on a subset of the children.
 
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