[css-scrollbars] Add alt text

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -217,7 +217,14 @@ Value Definitions</h3>
 	are colored.
 
 	<figure>
-		<img src="images/parts.svg" width=270 height=129>
+		<img src="images/parts.svg" width=270 height=129
+			alt="A vertical scrollbar with
+			a 'scroll-up' button at the top containing an upwards triangle,
+			a 'scroll-down' button at the top containing an downwards triangle,
+			a movable thumb indicating that the scrollar is all the way to the top,
+			all over a background track.
+			Each of these parts is labeled."
+			>
 		<figcaption>Anatomy of a scrollbar</figcaption>
 	</figure>