[css-multicol-2] Add ::column pseudo, per WG resolution. #10715 (#11186)

Author: tabatkins

css-multicol-2/Overview.bs

@@ -82,6 +82,48 @@ Stacking context</h3>
 
 	Issue: Add final content from previous level
 
+<h3 id=column-pseudo>
+Styling Columns: the ''::column'' pseudo-element</h3>
+
+The <dfn selector>::column</dfn> pseudo-element
+represents the individual columns
+in a [=multi-column container=].
+It only exists on [=multi-column containers=].
+
+A [=multi-column container=] has as many ''::column'' pseudo-elements
+as it has columns.
+They cannot be individually addressed;
+any styles applied to a multicol's ''::column'' pseudo-elements
+apply to all of them on that element.
+Each ''::column'' has the same size and position
+as its corresponding column:
+the same size as the column's [=available space=] in the [=inline axis=]
+and the container's [=content box=] in the [=block axis=].
+(It does not cover the gaps/rules between columns.)
+
+''::column'' pseudo-elements are treated as children of their [=multi-column container=],
+without any contents.
+They <em>do not</em> wrap the contents of the column,
+just fill the same space.
+
+The ''::column'' pseudo-element accepts only a very limited set of properties:
+
+* 'scroll-margin', 'scroll-snap-align', and 'scroll-snap-stop' 
+	(the scroll snap properties that apply to elements <em>inside</em> a [=scroll container=])
+
+Additionally, a ''::column'' pseudo-element can have a ''::scroll-marker'' pseudo-element of its own,
+as ''::column::scroll-marker''.
+(Other pseudo-elements do not exist on ''::column''.)
+Such ''::scroll-marker'' pseudo-elements
+inherit from the ''::column'' pseudo-element's [=superior parent=],
+rather than the ''::column'' itself.
+
+Note: This list of properties and pseudo-elements that work on ''::column''
+will likely be expanded in the future,
+as we develop more features
+that could usefully care about the <em>position</em> of a column
+without caring about the <em>contents</em>.
+
 <h2 id="column-gaps-and-rules">
 Column gaps and rules</h2>