[css-box-3] Change model of margin-trim. w3c#6643

Author: fantasai

css-box-4/Overview.bs

@@ -355,12 +355,12 @@ Margins at Container Edges: the 'margin-trim' property {#margin-trim}
 
 	<pre class="propdef">
 		Name: margin-trim
-		Value: none | in-flow | all
+		Value: none | block | inline | [ block-start || inline-start || block-end || inline-end ]
 		Initial: none
 		Applies to: [=block containers=], [=multi-column containers=]
 		Inherited: no
 		Percentages: N/A
-		Computed value: keyword as specified
+		Computed value: a set of zero to four keywords indicating which sides to trim
 		Animation type: discrete
 	</pre>
 
@@ -381,30 +381,40 @@ Margins at Container Edges: the 'margin-trim' property {#margin-trim}
 			child margins can collapse with their parent.
 			See <a href="https://www.w3.org/TR/CSS2/box.html#collapsing-margins">CSS2&sect;8.3.1: Collapsing Margins</a>.
 
-		<dt><dfn>in-flow</dfn>
+		<dt><dfn>block-start</dfn>
+		<dt><dfn>inline-start</dfn>
+		<dt><dfn>inline-end</dfn>
+		<dt><dfn>block-end</dfn>
 		<dd>
-			For in-flow boxes contained by this box,
-			block-axis margins adjacent to the box’s edges
+			For in-flow boxes and floats contained by this box,
+			margins adjacent to the box’s specified edges
 			are truncated to zero.
 			It also truncates any margins collapsed with such a margin.
 
-		<dt><dfn>all</dfn>
+		<dt><dfn>block</dfn>
 		<dd>
-			Trims the margins of in-flow boxes as for ''in-flow'',
-			but also trims any float margin
-			whose [=margin edge=] coincides
-			with the container’s [=content edge=].
+			Computes to ''block-start block-end''.
+
+		<dt><dfn>inline</dfn>
+		<dd>
+			Computes to ''inline-start inline-end''.
 	</dl>
 
-	Specifically, for [=block containers=],
-	''margin-trim: in-flow'' or ''margin-trim: all'' discards:
+	Note: Following the shortest-serialization principle,
+	[=computed values=] equivalent to ''margin-trim/block'' or ''margin-trim/inline''
+	will serialize as those keywords.
+
+	For [=block containers=] specifically,
+	'margin-trim' discards:
 	<ul>
-		<li>The block-start margin of a block-level first child.
-		<li>The block-end margin of a block-level last child.
+		<li>The block-start margin of a block-level first child,
+			when trimming at the block-start edge.
+		<li>The block-end margin of a block-level last child,
+			when trimming at the block-end edge.
 		<li>Any margin collapsed with these margins.
 	</ul>
 
-	''margin-trim: all'' also affects floats
+	It also affects floats
 	for which the [=block container=] is a [=containing block=] by:
 	<ul>
 		<li>
@@ -429,7 +439,7 @@ Margins at Container Edges: the 'margin-trim' property {#margin-trim}
 
 	ISSUE: Should this property apply to [=flex containers=] or [=grid containers=]?
 
-	ISSUE: Should floats have a <css>floats</css> value that only affects floats?
+	ISSUE(3256): Floats definition needs more work.
 
 	Note: See also the 'margin-break' property,
 	which applies to the box’s own margins