[css-align-3] Import the scrollable inline-block exception for determining baselines from 2.1. Fixes #2902.

Author: tabatkins

css-align-3/Overview.bs

@@ -2026,6 +2026,13 @@ Determining the Baselines of a Box</h2>
 			If there is no such line box or child,
 			then the block container has no [=baseline set=].
 
+			However, for legacy reasons,
+			if an ''inline-block''
+			is a [=scroll container=]
+			or contains no in-flow [=line boxes=],
+			its first and last [=baseline sets=]
+			are [=synthesized=] from its [=margin box=].
+
 		<dt>multi-column containers
 		<dd>
 			The first [=baseline set=] of a <a>multi-column container</a>