[css-align-3] Define multicol baselines as highest/lowest set. w3c#7856 w3c#7639

Author: fantasai

css-align-3/Overview.bs

@@ -2205,14 +2205,14 @@ Determining the Baselines of a Box</h2>
 		<dt>multi-column containers
 		<dd>
 			The first [=baseline set=] of a <a>multi-column container</a>
-			is taken from the first/last in-flow line box in the block container
-			or the first/last in-flow block-level child in the block container
-			that contributes a set of first/last baselines,
-			whichever comes first/last.
+			is the first [=baseline set=] of the [=column=] or [=spanner=]
+			with the highest ([=block-start=]&ndash;most) [=dominant baseline=].
 			If there is no such line box or child,
 			then the multi-column container has no first [=baseline set=].
 
-			Multi-column containers have no last [=baseline set=].
+			The last [=baseline set=] is analogous,
+			but uses the last [=baseline set=] from the [=column=] or [=spanner=]
+			with the <em>lowest</em> ([=block-end=]&ndash;most) [=dominant baseline=].
 
 		<dt>tables
 		<dd>