[css-multicol][css-display][css-contain] Becomes a formatting context (#1179)

* [css-contain][css-display] Move the definition of becoming a formatting context

* [css-multicol] spanners become a formatting context, not always a BFC

Closes #1071

Author: frivoal

css-contain/Overview.bs

@@ -320,15 +320,6 @@ Paint Containment</h3>
 	3. Because they are guaranteed to be stacking contexts,
 		scrolling elements can be painted into a single GPU layer.
 
-<h2 id="becoming-formatting-context">Becoming a formatting context</h2>
-
-When required by <a>layout containment</a> and <a>paint containment</a>,
-an element may need to <dfn lt='become a formatting context|becomes a formatting context'>become a <a>formatting context</a></dfn>.
-If the element already establishes a <a>formatting context</a> of any kind,
-this condition is satisfied.
-Otherwise, it is made to establish a <a>BFC</a> by changing its <a>inner display type</a> to ''flow-root''.
-
-
 Privacy and Security Considerations {#privsec}
 ==============================================
 

css-display/Overview.bs

@@ -739,6 +739,17 @@ Run-In Layout</h2>
 
 	Note: This run-in model is slightly different from the one proposed in earlier revisions of [[!CSS2]].
 
+<h2 id="becoming-formatting-context">Becoming a formatting context</h2>
+
+In some circumstances (See [[CSS-CONTAIN-1]] or [[CSS3-MULTICOL]] for examples),
+an element may need to <dfn export lt='become a formatting context|becomes a formatting context|becoming a formatting context'>become a <a>formatting context</a></dfn>.
+If the element already establishes a <a>formatting context</a> of any kind,
+this condition is satisfied.
+Otherwise, it is made to establish a <a>BFC</a> by changing its <a>inner display type</a> to ''flow-root''.
+This change happens at <a>used value</a> time,
+and does not affect the <a>computed value</a> of the 'display' property.
+
+
 <h2 id='glossary' class=no-num>
 Appendix A: Glossary</h2>
 
@@ -1102,6 +1113,7 @@ Changes</h2>
 		<li>Clarified interaction of run-ins with out-of-flow elements and ''::first-letter''.
 		<li>Switched ''table-caption'' and ''table-cell'' to use ''flow-root'' as their <a>inner display type</a>, since they always form a formatting context root.
 		<li>Closed off remaining issues and added at-risk list.
+		<li>Added the definition of <a>becoming a formatting context</a>
 	</ul>
 
 	Changes since the <a href="https://www.w3.org/TR/2015/WD-css-display-3-20150721/">21 July 2015 Working Draft</a> include:

css-multicol/Overview.bs

@@ -944,8 +944,8 @@ Spanning columns</h2>
 			ancestor in the same block formatting context.
 			Content in the normal flow that appears before the
 			element is automatically balanced across all columns before the
-			element appears. The element establishes a new block formatting
-			context.
+			element appears.
+			The element <a>becomes a formatting context</a>.
 
 			Note: The element establishing a formatting content does not
 			depend of whether the element is a descendent of a multicol or not.
@@ -1724,6 +1724,7 @@ This appendix describes changes from the
 <ul>
 	<li>TBD
 	<li>Clarify that column-span causes the element to become a formatting context even if it is not in a multicol
+	<li>Column spanners do not always establish a <em>block</em> formatting context, but instead <a>become a formatting context</a>.
 </ul>
 
 <h2 class=no-num id=acknowledgments>