[css-backgrounds-3] Dfn canvas surface and canvas background as terms. Mention that it's usually white. w3c#4863

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -1280,15 +1280,17 @@ 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, the surface that shows through is UA-dependent.
+<p>If the [=canvas background=] is not opaque,
+the <dfn export>canvas surface</dfn> below it shows through.
+The texture of the [=canvas surface=] is UA-dependent
+(but is typically an opaque white).
 
 <h4 id="root-background">
 The Canvas Background and the Root Element</h4>
 
-<p>The background of the root element becomes the background of the
-<a href="https://www.w3.org/TR/CSS2/intro.html#the-canvas">canvas</a>
+<p>The background of the root element becomes the <dfn export>canvas background</dfn>
 and its <span class=index>background painting area</span> extends
-to cover the entire canvas.
+to cover the entire <a href="https://www.w3.org/TR/CSS2/intro.html#the-canvas">canvas</a>.
 However, any images are sized and positioned relative to the root element
 as if they were painted for that element alone.
 (In other words, the