[css-align-3] Clarify when auto margins cause or interfere with coordinated self-alignment preference for baseline content-alignment. w3c#5923

Author: fantasai

css-align-3/Overview.bs

@@ -101,8 +101,9 @@ Module Interactions</h3>
 
 	This module adds some new alignment capabilities
 	to the block layout model described in [[!CSS2]] <a href="https://www.w3.org/TR/CSS2/visuren.html">chapters 9</a>
-	and <a href="https://www.w3.org/TR/CSS2/visudet.html">10</a>
-	and defines the interaction of these properties
+	and <a href="https://www.w3.org/TR/CSS2/visudet.html">10</a>,
+	<a href="#justify-block">redefines how overconstrained block-level box margins are resolved</a>,
+	and defines the interaction of these new alignment properties
 	with the alignment of table cell content using 'vertical-align',
 	as defined in [[!CSS2]] <a href="https://www.w3.org/TR/CSS2/tables.html#height-layout">chapter 17</a>.
 
@@ -1204,25 +1205,62 @@ Baseline Content-Alignment</h3>
 			and if its computed 'align-content' is ''first baseline''/''last baseline''.
 	</dl>
 
-	Additionally, if <a>self-alignment</a> applies to the box in an axis,
-	in order to participate in <a>baseline content-alignment</a> in that axis
-	it must also have a <dfn noexport>coordinated self-alignment preference</dfn>;
-	that is:
-
-	* The relevant <a>self-alignment property</a>
-		behaves as either ''align-self/stretch'' or ''self-start''/''self-end''
-		given a <a>baseline alignment preference</a> of ”first”/“last”.
-		For this purpose,
-		the ''start'', ''end'', ''flex-start'', and ''flex-end'' values
-		are treated as either ''self-start'' or ''self-end'',
-		whichever they end up equivalent to.
-	* Its used self-alignment is not affected by its [=overflow alignment=].
-		(That is, the used self-alignment is what would result from an ''unsafe'' [=overflow alignment=].)
-
-	<!-- This is because such a box is no longer guaranteed to line up the relevant edge with its siblings -->
+	Additionally,
+	in order to participate in <a>baseline content-alignment</a>
+	it must also have a <dfn noexport>coordinated self-alignment preference</dfn>,
+	to guarantee that the box lines up the relevant edge
+	with other boxes in its [=baseline-sharing group=].
+	That is, the box’s [=start=] ([=end=]) [=margin edge=]
+	must be intended to align--
+	and actually align--
+	to the corresponding [=edge=] of its [=containing block=]
+	when its [=baseline alignment preference=] is “first” (“last”).
 	It otherwise takes its <a>fallback alignment</a>.
 
-	See [[#align-by-baseline]] for exact details.
+	<details class="note">
+		<summary>When is a self-alignment preference coordinated?</summary>
+
+		A box’s [=start=] [=margin edge=] is aligned
+		to the corresponding edge of its [=containing block=]
+		and it has a [=coordinated self-alignment preference=]
+		for a “first” [=baseline alignment preference=] when,
+		in the relevant axis:
+
+		* There are no ''margin/auto'' [=margins=] and
+			the relevant <a>self-alignment property</a>
+			either is or aligns identically to
+			''align-self/stretch'' or ''self-start''; or
+		* There is only an ''margin/auto'' [=end=]-edge [=margin=],
+			which absorbs any positive free space
+			and disables the effects of any [=self-alignment property=],
+			<em>and</em>
+			its [=margin box=] does not overflow its [=containing block=]
+			under circumstances that would cause it
+			to effectively end-align instead
+			(such as having a [=containing block=] with an opposite [=writing mode=]).
+
+		A box’s [=end=] [=margin edge=] is aligned
+		to the corresponding edge of its [=containing block=]
+		and it has a [=coordinated self-alignment preference=]
+		for a “last” [=baseline alignment preference=] when,
+		in the relevant axis:
+
+		* There are no ''margin/auto'' [=margins=] and
+			the relevant <a>self-alignment property</a>
+			either is or aligns identically to ''self-end''
+			<em>and</em>
+			its self-alignment is what would result
+			from an ''unsafe'' [=overflow alignment=]; or
+		* There is only an ''margin/auto'' [=start=]-edge [=margin=],
+			which absorbs any positive free space
+			and disables the effects of any [=self-alignment property=]
+			<em>and</em>
+			its [=margin box=] does not overflow its [=containing block=]
+			under circumstances that would cause it
+			to effectively start-align instead.
+	</details>
+
+	See [[#align-by-baseline]] for additional details.
 	<a>Baseline content-alignment</a> can increase the intrinsic size of the box.
 
 <!--
@@ -2410,6 +2448,10 @@ Changes</h2>
 		(<a href="https://github.com/w3c/csswg-drafts/issues/3611">Issue 3611</a>)
 		<li>Clarified that only non-replaced boxes are affected by [=content-alignment].
 		(<a href="https://github.com/w3c/csswg-drafts/issues/4545">Issue 4545</a>)
+		<li>Clarified when a box has a [=coordinated self-alignment preference=]
+		that enables [=baseline content-alignment=],
+		particularly with respect to ''margin/auto'' [=margins=].
+		(<a href="https://github.com/w3c/csswg-drafts/issues/5923">Issue 5923</a>)
 	</ul>
 
 	Changes since the <a href="https://www.w3.org/TR/2018/WD-css-align-3-20181206/">6 December 2018 Working Draft</a> include: