[css-align] 3rd pass through gutters section: additional cleanup, plus define for flex layout.

Author: fantasai

css-align/Overview.bs

@@ -1617,20 +1617,13 @@ Row and Column Gutters: the 'row-gap' and 'column-gap' properties</h3>
 
 	These properties specify fixed-length <dfn export lt="gutter">gutters</dfn>
 	between items in the container,
-	adding space in a manner to the ''justify-content/space-between'' keyword
+	adding space between them--
+	in a manner to the ''justify-content/space-between'' keyword
 	of the <a>content-distribution properties</a>,
 	but of a fixed size instead of as a fraction of remaining space.
-
-	Different layout modes interpret these slightly differently:
-	for Multi-column Layout, it's a minimum spacing between column boxes;
-	for Flex Layout, it's a minimum spacing between <a>flex items</a>
-	and/or <a>flex lines</a>;
-	for Grid Layout, it's a minimum spacing between <a>grid tracks</a>.
-	Regardless of layout mode, however,
-	the directions that these two properties apply in are constant.
-	'column-gap' indicates spacing between “columns”--
-	it separates boxes in the container's <a>inline axis</a>,
-	similar to <a>inline-axis</a> 'margin'--
+	The 'column-gap' property specifies spacing between “columns”,
+	separating boxes in the container's <a>inline axis</a>
+	similar to <a>inline-axis</a> margin;
 	while 'row-gap' indicates spacing between “rows”,
 	separating boxes in the container's <a>block axis</a>.
 
@@ -1652,35 +1645,42 @@ Row and Column Gutters: the 'row-gap' and 'column-gap' properties</h3>
 			Issue: Should ''gap/normal'' just compute to ''1em'' in multicol?
 	</dl>
 
-	Note: Additional spacing may be added by 'justify-content'/'align-content'.
-	This space effectively increases the size of these <a>gutters</a>.
-
-<h4 id='gap-multicol'>
-Multicol Containers</h4>
-
-	'column-gap' specifes the [=gutter=] between adjacent <a>column boxes</a>.
-	See [[CSS-MULTICOL-1]] for details on how this affects the layout of multicol elements.
+	Gutters effect a minimum spacing between items:
+	additional spacing may be added by 'justify-content'/'align-content'.
+	Such additional space effectively increases the size of these <a>gutters</a>.
 
-	Multicol does not currently use 'row-gap'.
+	The exact handling of these properties varies by layout container:
 
-<h4 id='gap-flex'>
-Flex Containers</h4>
-
-	Issue: TODO define how row-gap/column-gap work between flex items and flex lines.
+	<dl>
+		<dt id="gap-multicol"><a>multi-column elements</a>
+		<dd>
+			'column-gap' specifes the [=gutter=] between adjacent <a>column boxes</a>.
+			See [[CSS-MULTICOL-1]] for details on how this affects the layout of multicol elements.
+			'row-gap' does not currently apply.
 
-<h4 id='gap-grid'>
-Grid Containers</h4>
+		<dt id="gap-flex"><a>flex containers</a>
+		<dd>
+			When applied to the <a>main axis</a>
+			(e.g. 'column-gap' in a ''flex-flow/row'' <a>flex container</a>),
+			indicates minimum spacing between items,
+			as if a fixed-size margin were inserted between adjacent <a>flex items</a>
+			in a single line.
 
-	The 'row-gap' and 'column-gap' properties
-	(and their 'gap' shorthand),
-	when specified on a <a>grid container</a>,
-	define the [=gutters=] between <a>grid rows</a> and <a>grid columns</a>.
-	See [[css-grid-1#gutters]] for precise details on how this works.
+			When applied to the <a>cross axis</a>
+			(e.g. 'row-gap' in a ''flex-flow/row'' <a>flex container</a>),
+			indicates minimum spacing between adjacent <a>flex lines</a>.
 
-<h4 id='gap-table'>
-Tables</h4>
+		<dt id="gap-grid"><a>grid containers</a>
+		<dd>
+			The 'row-gap' and 'column-gap' properties,
+			when specified on a <a>grid container</a>,
+			define the [=gutters=] between <a>grid rows</a> and <a>grid columns</a>,
+			respectively.
+			See [[css-grid-1#gutters]] for precise details.
+	</dl>
 
-	Table boxes do not use the 'gap' properties to specify separation between their cells.
+	Note: Table boxes do not use the 'gap' properties
+	to specify separation between their cells.
 	Instead, they use the 'border-spacing' property,
 	which has slightly different functionality:
 	it inherits,