[css-multicol-1] Spanners and margins. Fixes #2203

Author: rachelandrew

css-multicol-1/Overview.bs

@@ -1092,6 +1092,11 @@ h2 {
 		</figure>
 	</div>
 
+	Spanners are block-level boxes 
+	therefore the margins of two adjacent spanners will collapse with each other.
+	As column boxes establish a new [=block formatting context=],
+	margins on elements inside a column box will not collapse with the margin of a spanner.
+
 	<div class="example">
 		Spanners establish new [=formatting contexts=], but their margins can be changed by their surroundings. 
 		In this example, two spanners naturally end up at the top of a page. 
@@ -1439,6 +1444,10 @@ This appendix is <em>informative</em>.
 
 <h3 id="changes-from-20180528">Changes from the <a href="https://www.w3.org/TR/2018/WD-css-multicol-1-20180528/">Working Draft (WD) of 28 May 2018</a></h3>
 <ul>
+	<li>Added the paragraph, "Spanners are block-level boxes 
+	therefore the margins of two adjacent spanners will collapse with each other.
+	As column boxes establish a new [=block formatting context=],
+	margins on elements inside a column box will not collapse with the margin of a spanner." <a href="https://github.com/w3c/csswg-drafts/issues/2203#issuecomment-431783027">Resolved 22 Oct 2018</a>, see also the resolution for <a href="https://github.com/w3c/csswg-drafts/issues/2582#issuecomment-402619872">issue 2582</a>.</li>
 	<li>Clarified the spec to explain that a spanning element is taken out-of-flow, leaving a forced break. Added the paragraph, "A spanning element is taken out-of-flow, leaving a forced break. This does not affect the painting order of the spanning element." <a href="https://github.com/w3c/csswg-drafts/issues/1072#issuecomment-468087733">Resolved 28 Feb 2019</a>.</li>
 	<li>Moved the definition of the 'column-gap' property to [[CSS3-ALIGN]] and added a paragraph detailing the specifics of 'column-gap' in multicol:
 	<br />"In a multi-column formatting context the used value of normal for the column-gap property is 1em. This ensures columns are readable when the initial values are used. If there is a column rule between columns, it will appear in the middle of the gap." <a href="https://github.com/w3c/csswg-drafts/issues/3641">Resolved 4 June 2019.</a></li>