[csswg] Add comment about what pre.output actually does.

--HG--
extra : rebase_source : fd50fd3859486384e3daf34ee059d6547230f38f

Author: tabatkins

default.css

@@ -195,6 +195,7 @@
 	pre {
 		margin-top: 1em;
 		margin-bottom: 1em;
+		overflow: auto;
 	}
 
 /** Example Code **************************************************************/
@@ -407,6 +408,9 @@
 		margin: 1em auto;
 	}
 
+	/* Displaying the output of text layout, particularly when
+	   line-breaking is being invoked, and thus having a
+	   visible width is good. */
 	pre.output {
 		margin: 1em;
 		border: solid thin silver;