[css-backgrounds-3] Use 'surface' to distinguish canvas background from what lies underneath. w3c#3949

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -1279,7 +1279,7 @@ then the canvas background is transparent.
 For example, if the element has ''visibility: hidden'' but not ''display: none'',
 boxes <em>are</em> generated for it and its background <em>is</em> used for the canvas.
 
-<p>If the canvas background is not opaque, what shows through is UA-dependent.
+<p>If the canvas background is not opaque, the surface that shows through is UA-dependent.
 
 <h4 id="root-background">
 The Canvas Background and the Root Element</h4>