[css-overflow-4] Make example more robust

Author: frivoal

css-overflow-4/Overview.bs

@@ -637,6 +637,7 @@ ellipsis interaction with scrolling interfaces</h3>
 		div.crawlbar {
 			text-overflow: ellipsis;
 			height: 2em;
+			line-height: 1em;
 			overflow: scroll;
 			white-space: nowrap;
 			width: 15em;
@@ -654,7 +655,7 @@ ellipsis interaction with scrolling interfaces</h3>
 		</code></pre>
 
 		demonstration of sample CSS and HTML:
-		<div style="text-overflow: ellipsis; height: 2em; overflow: scroll; white-space: nowrap; width: 15em; border:1em solid black;">
+		<div style="text-overflow: ellipsis; height: 2em; line-height:1; overflow: scroll; white-space: nowrap; width: 15em; border:1em solid black;">
 		CSS is awesome, especially when you can scroll
 		to see extra text instead of just
 		having it overlap other text by default.