[css-align] Define table column baselines

Author: fantasai

css-align/Overview.bs

@@ -897,7 +897,8 @@ Determining the Baseline of a Box</h2>
 
 	The <dfn>first baseline</dfn> (and <dfn>last baseline</dfn>) of a box
 	for a given axis
-	is a set of baselines (alphabetic, central, etc.) belonging to a line box within the box.
+	is a set of baselines (alphabetic, central, etc.)
+	nominally associated with a 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>.
 	(Note, however, that boxes might not have a baseline in a particular axis.)
@@ -924,7 +925,8 @@ Determining the Baseline of a Box</h2>
 		<dd>
 			The first (last) inline-axis baseline of a table box is the baseline of its first (last) row.
 			However, when finding the baseline of an inline-block,
-			table boxes must be skipped.
+			any baselines contributed by the table boxes must be skipped.
+			(This quirk is a legacy behavior from [[CSS21]].)
 
 			The block-axis baseline of a table box is the baseline of its first (last) column.
 
@@ -933,7 +935,7 @@ Determining the Baseline of a Box</h2>
 
 		<dt>''table-row''
 		<dd>
-			If any cells in the row participate in ''baseline'' (''last-baseline'') alignment,
+			If any cells in the row participate in ''baseline'' (''last-baseline'') alignment along the row axis,
 			the first (last) inline-axis baseline of the row is
 			generated from their shared <a>alignment baseline</a> and the row's <a>first available font</a>,
 			after alignment has been performed.
@@ -943,6 +945,19 @@ Determining the Baseline of a Box</h2>
 
 			A table row has no block-axis baseline.
 
+		<dt>''table-column''
+		<dd>
+			If any cells in the column participate in ''baseline'' (''last-baseline'') alignment
+			along the column axis (by having a writing mode perpendicular to that of the table),
+			the first (last) block-axis baseline of the column is
+			generated from their shared <a>alignment baseline</a> and the column's <a>first available font</a>,
+			after alignment has been performed.
+			Otherwise, the the first (last) inline-axis baseline of the row
+			is synthesized from the extreme content edges of the cells in the row.
+			[[!CSS21]]
+			
+			A table column has no inline-axis baseline.
+
 		<dt>''table-cell''
 		<dd>
 			The inline-axis baseline of a table cell is the baseline of the first (last) in-flow line box in the cell,