[css-overflow-3] w3c#3779 Do not propagate overflow from boxless elements. (w3c#4148)

* [css-overflow-3] Do not propagate overflow from boxless elements.

Edits for w3c#3779.

* applied -> propagated

Co-Authored-By: Florian Rivoal <git@florian.rivoal.net>

* canvas -> viewport

Co-Authored-By: Florian Rivoal <git@florian.rivoal.net>

Author: lilles

css-overflow-3/Overview.bs

@@ -489,6 +489,11 @@ Overflow Viewport Propagation</h3>
 	The element from which the value is propagated must then have
 	a used 'overflow' value of ''overflow/visible''.
 
+	The overflow values are not propagated to the viewport if no boxes are
+	generated for the element whose overflow values
+	would be used for the viewport (for example, if the root element has
+	display: none).
+
 	If ''overflow/visible'' is applied to the viewport,
 	it must be interpreted as ''overflow/auto''.
 	If ''overflow/clip'' is applied to the viewport,