Rework baselines a bit. Things can have baselines in either axis, and we distinguish between 'natural' and 'synthetic' baselines.

Author: tabatkins

css-align/Overview.bs

@@ -283,28 +283,32 @@ Baseline Alignment: the ''baseline'' and ''last-baseline'' keywords</h3>
 		<dt><dfn>baseline</dfn>
 		<dd>
 			Indicates <dfn dfn for export>first-baseline alignment</dfn>:
-			aligns one of baselines in the box's <a>first baseline set</a>
-			in the appropriate axis
+			If the box has a <a>natural baseline</a> in the appropriate axis,
+			aligns one of the baselines in the box's <a>first baseline set</a>
+			in that axis
 			with the corresponding baseline
 			in the <a>first baseline sets</a>
 			of all the boxes in its <a>baseline-sharing group</a>.
 
 			If the <a>alignment subject</a>’s position is not fully determined by <a>baseline alignment</a>,
-			the content is ''start''-aligned insofar as possible while preserving the baseline alignment.
-			(Content that has no <a>first baselines</a> is thus start-aligned.)
+			or the box has no <a>natural baseline</a> in the appropriate axis,
+			the content is ''start''-aligned insofar as possible while preserving the <a>baseline alignment</a>.
+			(<a>Alignment subjects</a> that have no inline-level content are thus ''start''-aligned.)
 
 		<dt><dfn>last-baseline</dfn>
 		<dd>
 			Indicates <dfn dfn for export>last-baseline alignment</dfn>:
-			aligns one of baselines in the box's <a>last baseline set</a>
-			in the appropriate axis
+			If the box has a <a>natural baseline</a> in the appropriate axis,
+			aligns one of the baselines in the box's <a>last baseline set</a>
+			in that axis
 			with the corresponding baseline
 			in the <a>last baseline sets</a>
 			of all the boxes in its <a>baseline-sharing group</a>.
 
 			If the <a>alignment subject</a>’s position is not fully determined by <a>baseline alignment</a>,
-			the content is ''end''-aligned insofar as possible while preserving the baseline alignment.
-			(Content that has no <a>last baselines</a> is thus end-aligned.)
+			or the box has no <a>natural baseline</a> in the appropriate axis,
+			the content is ''end''-aligned insofar as possible while preserving the <a>baseline alignment</a>.
+			(<a>Alignment subjects</a> that have no inline-level content are thus ''end''-aligned.)
 
 			Issue: Should this ''end'' fallback be ''safe'' or ''unsafe''?
 	</dl>
@@ -677,7 +681,7 @@ Baseline Content-Alignment</h3>
 		<dt>Table Cells:
 		<dd>
 			A <a>table cell</a> participates in first (last) <a>baseline content-alignment</a>
-			in either its row or column (whichever matches its <a>inline axis</a>)
+			in either its row or column (whichever matches the axises of its <a>natural baselines</a>)
 			if its computed 'align-content' is ''baseline'' (''last-baseline'').
 
 		<dt>Flex Items:
@@ -695,7 +699,7 @@ Baseline Content-Alignment</h3>
 		<dt>Grid Items:
 		<dd>
 			A <a>grid item</a> participates in first (last) <a>baseline content-alignment</a>
-			in either its row or column (whichever matches its <a>inline axis</a>)
+			in either its row or column (whichever matches the axises of its <a>natural baselines</a>)
 			if its computed 'align-content' is ''baseline'' (''last-baseline''),
 			and its computed 'align-self' or 'justify-self' (whichever affects its <a>block axis</a>)
 			is ''align-self/stretch'' or ''self-start'' (''self-end'').
@@ -1148,7 +1152,7 @@ Baseline Self-Alignment</h3>
 		<dt>Grid Items:
 		<dd>
 			A <a>grid item</a> participates in first (last) <a>baseline self-alignment</a>
-			in either its row or column (whichever matches its <a>inline axis</a>)
+			in either its row or column (whichever axises it has a <a>natural baseline</a> in)
 			if its 'justify-self' or 'align-self' property (whichever affects its <a>block axis</a>)
 			computes to ''baseline'' (''last-baseline'').
 	</dl>
@@ -1363,6 +1367,14 @@ Determining the Baselines of a Box</h2>
 
 	Note: The forthcoming Inline Layout Module will define synthesis rules for baselines other than alphabetic and central.
 
+	A box has a <dfn export>natural baseline</dfn>
+	if it derives its <a>baseline set</a>(s) from a font's baseline table
+	(whether or not the UA had to <a href="https://drafts.csswg.org/css-inline/#baseline-synthesis-fonts">synthesize that baseline table</a>,
+	e.g. due to it being missing from the font).
+	It has a <dfn export>synthesized baseline</dfn>
+	if its baselines are <a>synthesized</a> from the geometry of its box,
+	as described above.
+
 	<p class='issue'>
 		Maybe these things are wrong?
 		CSS 2.1 is really weird about baseline alignment.
@@ -1375,10 +1387,12 @@ 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> whose axis is parallel to an axis in which they have a baseline
+		<li>the baselines that want to align are on the same “side” of the alignment context,
+		e.g. the boxes in a vertical-axis <a>alignment context</a> all want to align their leftmost baseline
+		(whether that is the first baseline in a ''vertical-lr'' flow,
+		the last baseline in a ''vertical-rl'' flow,
+		or the first baseline of a ''vertical-lr'' flow that has been propagated to being the block-axis <a>first baseline</a> of a ''horizontal-tb'' ''ltr'' grid container)
 	</ul>
 
 	Boxes <dfn lt="shared alignment context|alignment context" export>share an alignment context</dfn> along a particular axis when they are:
@@ -1391,6 +1405,9 @@ Baseline Alignment Terminology</h3>
 		<li>flex items in the same flex line, along the flex container's main axis
 	</ul>
 
+	Note: The inline-level boxes on a line box also share an alignment context and participate in a baseline-sharing group;
+	however this is detailed in the CSS Inline Layout module, not here.
+
 	Boxes in a <a>baseline-sharing group</a> are aligned to each other
 	using their <dfn export>alignment baseline</dfn>.
 	This is the dominant baseline associated with
@@ -1400,6 +1417,9 @@ Baseline Alignment Terminology</h3>
 	specifying ''align-content: baseline'' on table cells in the same row
 	will align the alphabetic baselines of their <a>first formatted lines</a>.
 
+	Note: The 'alignment-baseline' property introduced in CSS Inline Layout Level 3
+	will allow changing which baseline is used for baseline alignment within a <a>baseline-sharing group</a>.
+
 <h3 id='align-by-baseline'>
 Aligning Boxes by Baseline</h3>
 

css-flexbox/Overview.bs

@@ -2007,8 +2007,8 @@ Cross-axis Alignment: the 'align-items' and 'align-self' properties</h3>
 
 		<dt><dfn>baseline</dfn>
 		<dd>
-			If the <a>flex item</a>’s inline axis is the same as the <a>cross axis</a>,
-			this value is identical to <a value for=align-self>flex-start</a>.
+			If the <a>flex item</a> has no <a>natural baselines</a> in its <a>main axis</a>,
+			this value is identical to ''align-self/flex-start''.
 
 			Otherwise,
 			it <dfn dfn id='baseline-participation'>participates in baseline alignment</dfn>:
@@ -2178,7 +2178,8 @@ Flex Container Baselines</h3>
 				is <a lt="generate baselines">generated</a> from
 				its <a>alignment baseline</a>.
 
-			3. Otherwise, the flex container's first/last <a>main-axis baseline set</a>
+			3. Otherwise, the flex container has no natural first/last main-axis <a>baseline set</a>,
+				and a synthesized first/last <a>baseline set</a>, if needed,
 				is <a lt="synthesize baselines">synthesized</a>
 				from the first/last item's content box,
 				or, failing that, from the flex container's content box.
@@ -2200,7 +2201,8 @@ Flex Container Baselines</h3>
 				the flex container's first/last <a>cross-axis baseline set</a>
 				is that baseline set.
 
-			2. Otherwise, the flex container's <a>cross-axis baseline set</a>
+			2. Otherwise, the flex container has no natural cross-axis <a>baseline set</a>,
+				and a synthesized first/last <a>baseline set</a>, if needed,
 				is <a lt="synthesize baselines">synthesized</a>
 				from the first/last item's content box,
 				or, failing that, from the flex container's content box.

css-grid/Overview.bs

@@ -3516,17 +3516,14 @@ Grid Container Baselines</h3>
 			that <a>grid item</a>’s <a>alignment baseline</a>.
 
 		<li>
-			Otherwise, the grid container's first (last) baseline set
+			Otherwise, the grid container has no natural first (last) <a>baseline set</a> in that axis,
+			and a synthesized first/last <a>baseline set</a>, if needed,
 			is <a lt="synthesize baselines">synthesized</a>
 			from the first (last) item's (in <a>grid order</a>) content box,
 			or, if there's no <a>grid item</a>,
 			from the grid container's content box.
 	</ol>
 
-	A <a>grid item</a> <dfn export id="baseline-participation">participates in baseline alignment</dfn> in a particular dimension
-	if its value for 'align-self' or 'justify-self', as appropriate, is ''baseline''
-	and its inline axis is parallel to that dimension.
-
 	<dfn export lt="grid order|grid-modified document order">'grid-modified document order (grid order)</dfn>
 	is the order in which <a>grid items</a> are encountered
 	when traversing the grid's <a>grid cells</a>,

css-inline/Overview.bs

@@ -160,8 +160,10 @@ Alignment Point: 'alignment-baseline' longhand</h4>
 	<p>Specifies what point of an inline-level box is aligned to what point in the parent.
 	Values are defined below:
 
-	For the following definitions, the margin box is used for atomic inlines,
-	and the leading box for non-replaced inlines:
+	For the following definitions,
+	the margin box is used for atomic inlines,
+	the leading box for non-replaced inlines,
+	and the baselines of the box are <a lt="synthesized baseline">synthesized</a> if missing in the line-box's inline axis:
 
 	<dl dfn-for=alignment-baseline dfn-type=value>
 		<dt><dfn>baseline</dfn>