[css-multicol] Removing sentences about columns as block container boxes, adding clarification and an example. Fixes w3c#1738

Author: rachelandrew

css-multicol-1/Overview.bs

@@ -325,11 +325,27 @@ The multi-column model</h2>
 	slightly.
 -->
 
-	Column boxes are block container boxes. That
-	is, column boxes behave like block-level, table cell, and inline-block
-	boxes as per CSS 2.1, section 10.1, item 2 [[!CSS21]]. However, column
-	boxes do not establish block container boxes for elements with
-	''position: fixed'' or ''position: absolute''.
+	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 element, it being the principal box.
+
+	<div class=example>
+		In this example, the multi-column container has ''position: relative'' thus becoming the containing block. The image is a direct child of the multi-column container and has ''position: absolute''. It takes positioning from the multi-column container and not from the column box.
+
+		<pre highlight="css">
+		.container {
+			position: relative;
+			column-count: 3;
+		}
+		img { 
+			position: absolute;
+			top: 20px;
+			left: 40px;
+		}
+		</pre>
+
+		<img src="images/column-not-containing-block.svg" alt="The absolutely positioned image is positioned by reference to the multicol container not the column box.">
+	</div>
 
 	<div class=example>
 		In this example, the width of the image is set with these rules:

css-multicol-1/images/column-not-containing-block.svg

@@ -133,7 +133,8 @@ Comment:  https://lists.w3.org/Archives/Public/www-style/2011Dec/0058.html
 Comment:  https://lists.w3.org/Archives/Public/www-style/2012Jan/0392.html
 Comment:  https://lists.w3.org/Archives/Public/www-style/2013Apr/0338.html
 Comment:  https://github.com/w3c/csswg-drafts/issues/1738
-Open
+Closed:   Accepted 
+Resolved: https://github.com/w3c/csswg-drafts/issues/1738#issuecomment-342661881
 ----
 Issue 14.
 Summary:  z-index and pseudo stacking contexts, column rules are painted in the inline content layer