[css-align] Make orthogonal flows synthesize baselines and behave like replaced elements.

Author: fantasai

css-align/Overview.bs

@@ -1439,7 +1439,7 @@ Determining the Baselines of a Box</h2>
 	nominally associated with the first (last) line of text within the box.
 	The <a>alignment baseline</a> is one of these,
 	usually the dominant baseline of the <a>alignment container</a>.
-	(See 'alignment-baseline'.)
+	(See the 'alignment-baseline' property in [[CSS-INLINE-3]].)
 	Note that boxes might not have baselines in a particular axis.
 
 	The first and last <a>baseline sets</a> of a box
@@ -1548,12 +1548,21 @@ Baseline Alignment Terminology</h3>
 	This is possible only if they
 
 	<ul>
-		<li>share an <a>alignment context</a> whose axis is parallel to their <a>inline axis</a>
-		<li>either have the same <a>block flow direction</a> and <a>baseline alignment preference</a>,
-			or have opposite <a>block flow direction</a> and opposite <a>baseline alignment preference</a>
-			(in other words, the baselines that want to align are on the same side of the alignment context).
+		<li>Share an <a>alignment context</a> along an axis perpendicular to their baseline alignment axis.
+		<li>Have compatible <a>baseline alignment preferences</a>
+		(i.e., the baselines that want to align are on the same side of the alignment context).
+
 	</ul>
 
+	The <a>baseline alignment preferences</a> of two boxes are compatible if they have the same <a>block flow direction</a> and <a>baseline alignment preference</a>,
+	or have opposite <a>block flow direction</a> and opposite <a>baseline alignment preference</a>.
+	For this purpose, boxes that establish an <a>orthogonal flow</a>
+	are treated as having the same <a>block flow direction</a>
+	as their <a>alignment context</a>,
+	and their baselines are <a>synthesized</a> accordingly,
+	exactly as for replaced elements and other boxes that have no <a>natural baseline</a>.
+
+
 	Boxes <dfn lt="shared alignment context|alignment context" export>share an alignment context</dfn> along a particular axis when they are:
 
 	<ul>