[css-display] Define 'formatting context' and remove issue.

Author: tabatkins

css-display/Overview.bs

@@ -741,9 +741,23 @@ Appendix A: Glossary</h2>
 
 		<dt><dfn>formatting context</dfn>
 		<dd>
-			The thing that makes CSS do the layouts.
-
-			Issue: wordsmith this.
+			A <a>formatting context</a> is an independent "unit" of layout.
+			When a box establishes a <a>formatting context</a>,
+			the layout of its descendants are not affected by any outside boxes,
+			and vice versa.
+
+			<p class="example">
+				For example, when used among other ''flow'' boxes,
+				floated boxes affect the layout of surrounding boxes.
+				But their effects do not escape their <a>formatting context</a>:
+				the box establishing their <a>formatting context</a> grows to fully contain them,
+				and any other <a>formatting contexts</a> near them
+				shrink or move to avoid having the float overlap them.
+
+			Most <a>inner display types</a> establish formatting contexts automatically.
+			The ''flow'' layout mode is a notable exception,
+			with a choice between establishing a formatting context or not,
+			via the ''flow-root'' and ''flow'' 'display' values.
 
 		<dt><dfn>block formatting context</dfn>
 		<dt><dfn>inline formatting context</dfn>