[css-box-4] Apply margin-trim to flex/grid. w3c#3255

Author: fantasai

css-box-4/Overview.bs

@@ -357,7 +357,7 @@ Margins at Container Edges: the 'margin-trim' property {#margin-trim}
 		Name: margin-trim
 		Value: none | block | inline | [ block-start || inline-start || block-end || inline-end ]
 		Initial: none
-		Applies to: [=block containers=], [=multi-column containers=]
+		Applies to: [=block containers=], [=multi-column containers=], [=flex containers=], [=grid containers=]
 		Inherited: no
 		Percentages: N/A
 		Computed value: a set of zero to four keywords indicating which sides to trim
@@ -404,6 +404,22 @@ Margins at Container Edges: the 'margin-trim' property {#margin-trim}
 	[=computed values=] equivalent to ''margin-trim/block'' or ''margin-trim/inline''
 	will serialize as those keywords.
 
+	Adjacency is not sensitive to space governed by box alignment [[!CSS-ALIGN-3]],
+	and ignores collapsed boxes (see [[css-flexbox-1#visibility-collapse]])
+	and tracks ([[css-grid-1#auto-repeat]]).
+
+	Note: See also the 'margin-break' property,
+	which applies to the box’s own margins
+	when they adjoin a <a>fragmentation break</a>
+	(page break / column break / etc.).
+
+	ISSUE: Define how this property affects margins at breaks
+	if the box establishes a <a>fragmentation context</a>.
+	See also <a href="https://github.com/w3c/csswg-drafts/issues/3314">Issue 3314</a>.
+
+<h4 id="margin-trim-block">
+Trimming Block Container Content</h4>
+
 	For [=block containers=] specifically,
 	'margin-trim' discards:
 	<ul>
@@ -437,18 +453,32 @@ Margins at Container Edges: the 'margin-trim' property {#margin-trim}
 			of a [=block formatting context root=].
 	</ul>
 
-	ISSUE: Should this property apply to [=flex containers=] or [=grid containers=]?
-
 	ISSUE(3256): Floats definition needs more work.
 
-	Note: See also the 'margin-break' property,
-	which applies to the box’s own margins
-	when they adjoin a <a>fragmentation break</a>
-	(page break / column break / etc.).
+<h4 id="margin-trim-flex">
+Trimming Flex Container Content</h4>
 
-	ISSUE: Define how this property affects margins at breaks
-	if the box establishes a <a>fragmentation context</a>.
-	See also <a href="https://github.com/w3c/csswg-drafts/issues/3314">Issue 3314</a>.
+	For [=flex containers=] specifically,
+	'margin-trim' discards
+	<ul>
+		<li>the corresponding margin of each [=flex item=] on the closest [=flex line=]
+			when trimming an edge parallel to the [=main axis=]
+		<li>the corresponding margin of the first/last [=flex item=] on each [=flex line=]
+			when trimming an edge parallel to the [=cross axis=]
+	</ul>
+
+	This process ignores any [=collapsed flex items=].
+
+<h4 id="margin-trim-grid">
+Trimming Grid Container Content</h4>
+
+	For [=grid containers=] specifically,
+	'margin-trim' discards
+	the corresponding margin of each [=grid item=]
+	in the [=grid track=] adjacent to the relevant [=box edge=].
+
+	This process ignores any [=collapsed grid tracks=].
+	It does not otherwise ignore any empty [=grid tracks=].
 
 Padding {#paddings}
 =======