[CSS21] New text for erratum for canvas background. Still needs validation.

Author: bert-github

css2/colors.src

@@ -84,13 +84,14 @@ for the root element.
 <!-- ToDo: ask the WG to validate the following text for erratum
 http://www.w3.org/Style/css2-updates/REC-CSS2-20110607-errata.html#s.14.2 -->
 
-<p>However, if the element whose background would be used for the
-background of the canvas has 'display: none', or is a descendant of an
-element with 'display: none', then the canvas background is
-undefined.
+<p>However, if no boxes are generated for the element whose background
+would be used for the background of the canvas, then the canvas
+background is transparent. (in CSS 2.1, that is the case when the
+element or an ancestor has 'display: none'.)
 
 <p>Note that, if the element has 'visibility: hidden' but not
-'display: none', its background <em>is</em> used for the canvas.
+'display: none', boxes <em>are</em> generated for it and its
+background <em>is</em> used for the canvas.
 
 <!-- end of erratum -->