[css-multicol] Pass at incorporating the new model plus clarification in the issue re naming (w3c#3999)

* Trying to incorporate the new model

* Tidying up section about gaps

The gap is the same size in all rows and lines in the container.

* update to PR based on discussion at F2F

* Fixing up the multicol model

Author: rachelandrew

css-multicol-1/Overview.bs

@@ -149,41 +149,77 @@ Introduction</h2>
 <h2 id="the-multi-column-model">
 The multi-column model</h2>
 
-	A <dfn lt="multi-column container|multicol container" oldids="multi-column-element" export>multi-column container</dfn>
-	(or <i>multicol container</i> for short)
-	is an element whose 'column-width' or 'column-count'
-	property is not ''column-width/auto'' and therefore acts as a container for
-	multi-column layout.
-
-	<wpt>
-	multicol-count-computed-004.xht
-	</wpt>
-
-	In the traditional CSS box model, the content of an element is
-	flowed into the content box of the corresponding element. Multi-column
-	layout introduces a new type of container between the content box and
-	the content, namely the <dfn export local-lt=column>column box</dfn> (or <i>column</i> for
-	short). The content of a multicol container is flowed into its column
-	boxes.
-
-	Column boxes in a multi-column container are arranged into <dfn export lt="column row" local-lt=row>rows</dfn>.
-	Like table cells,
-	the column boxes in a row are ordered
-	in the inline direction of the multicol container.
-	The <dfn export>column width</dfn> is the length of the column box in the inline direction.
+    An element whose 'column-width' or 'column-count' property is not ''column-width/auto'' 
+	establishes a <dfn lt="multi-column container|multicol container" oldids="multi-column-element" export>multi-column container</dfn>
+	(or <i>multicol container</i> for short),
+	and therefore acts as a container for [=multi-column layout=].
+
+	In the traditional CSS box model, 
+	the content of an element is
+	flowed into the content box of the corresponding element. 
+	Multi-column layout introduces a [=fragmentation context=]
+	formed of <a lt="anonymous box">anonymous</a> [=fragmentation containers=] 
+	called <dfn export lt="column box" local-lt=column>column boxes</dfn> 
+	(or <i>columns</i> for short).
+	These [=column boxes=] establish
+	an independent [=block formatting context=]
+	into which the multi-column container's content flows,
+	and form the [=containing block=] for its non-positioned childen.
+	Content overflowing a [=column box=] in the [=block axis=]
+	[=fragments=] and continues in the next [=column box=].
+
+	Note: Column boxes, which are [=anonymous boxes=],
+	do not become the containing block
+	for [=absolutely-positioned boxes=].
+	The 'position' property, which establishes a containing block for such boxes,
+	applies to the [=multicol container=], it being the [=principal box=].
+
+	The column boxes are ordered
+	in the [=inline base direction=] of the multicol container
+	and arranged into <dfn lt="multi-column line | multi-col line">multicol lines</dfn>.
+    The <dfn export>column width</dfn> is the length of the column box in the inline direction.
 	The <dfn export>column height</dfn> is the length of the column box in the block direction.
-	All column boxes in a row have the same column width,
-	and all column boxes in a row have the same column height.
-	Within each row in the multi-column container,
+	All column boxes in a line have the same column width,
+	and all column boxes in a line have the same column height.
+
+	Within each [=multi-col line=] in the multi-column container,
 	adjacent column boxes are separated by a <dfn noexport>column gap</dfn>,
 	which may contain a <dfn noexport>column rule</dfn>.
-	All column gaps in the same row are equal.
-	All column rules in the same row are also equal, if they appear;
+	All column gaps in the same multi-column container are equal.
+	All column rules in the same multi-column container are also equal, if they appear;
 	column rules only appear between columns that both have content.
 
-	In the simplest case a multicol container will contain only one row
+	In the simplest case a multicol container will contain only one line
 	of columns, and the height of each column will be equivalent to the
 	used height of the multi-column container's content box.
+	However, [=fragmentation=] or [=spanners=]
+	can split the content of the [=multi-column container=]
+	into multiple [=multi-col lines=].
+
+	If the multi-column container is paginated, the height of each column is
+	constrained by the page and the content continues in a new line of
+	column boxes on the next page; a column box never splits across pages.
+
+	The same effect occurs when a <i>spanning element</i> divides the
+	multi-column container: the columns before the spanning element are
+	balanced and shortened to fit their content. Content after the
+	spanning element then flows into a new, subsequent line of column boxes.
+
+	A [=multi-column container=] therefore is a regular [=block container=]
+	that establishes a new [=independent formatting context=]
+	whose contents consist of a series of
+	[=multi-col lines=] and [=multi-col spanners=].
+	Each [=multi-column line=] acts as a [=block-level box=]
+	that establishes a [=multi-column formatting context=]
+	for its [=column boxes=];
+	and each [=spanner=] acts as a [=block-level box=]
+	that establishes an [=independent formatting context=]
+	with its type depending on its 'display' value as usual.
+
+	<wpt>
+	multicol-count-computed-004.xht
+	</wpt>
+
 
 	<div class="example">
 		Column gaps (diagonal hatching) and column rules are shown in this
@@ -205,25 +241,13 @@ The multi-column model</h2>
 		</figure>
 	</div>
 
-	If the multi-column container is paginated, the height of each row is
-	constrained by the page and the content continues in a new row of
-	column boxes on the next page; a column box never splits across pages.
-
-	The same effect occurs when a <i>spanning element</i> divides the
-	multi-column container: the columns before the spanning element are
-	balanced and shortened to fit their content. Content after the
-	spanning element then flows into a new row of column boxes.
-
 	<div class="example">
 		<figure>
 			<img alt="a diagram showing a spanning element causing the shortened columns above the element with text continuing in new columns below" src="images/simple-span-example.svg">
 			<figcaption>A demonstration of how the spanning element divides the multicol container.</figcaption>
 		</figure>
 	</div>
 
-	Column boxes are block container boxes. The multi-column container is the principal box, and column boxes are anonymous.
-
-	Note: Column boxes do not become the containing block for elements with ''position: fixed'' or ''position: absolute''. The containing block is the multicol container, it being the principal box.
 
 	<wpt>
 	multicol-containing-001.xht