[css-ui-4] Interaction with user-select:none elements does not affect the selection

Closes w3c#319

Author: frivoal

css-ui-4/Overview.bs

@@ -715,6 +715,10 @@ Omitted values are set to their initial values.
 		Copying text that does not appear to be selected, or vice versa,
 		is highligh confusing to users.</span>
 
+		Attempting to start a selection in an element where 'user-select' is ''user-select/none'',
+		such as by clicking in it or starting a drag in it,
+		must not cause a pre-existing selection to become unselected or to be affected in any way.
+
 		As 'user-select' is a UI convenience mechanism,
 		not a copy protection mechanism,
 		the UA may provide an alternative way for the user