Added sync= attr to DOMWidget

IPython/html/widgets/widget.py

@@ -330,12 +330,10 b' class Widget(LoggingConfigurable):'
 
 
 class DOMWidget(Widget):
-    visible = Bool(True, help="Whether or not the widget is visible.")
+    visible = Bool(True, help="Whether or not the widget is visible.", sync=True)
 
     # Private/protected declarations
-    _css = Dict() # Internal CSS property dict
-
-    keys = ['visible', '_css'] + Widget.keys # TODO
+    _css = Dict(sync=True) # Internal CSS property dict
 
     def get_css(self, key, selector=""):
         """Get a CSS property of the widget.
@@ -355,7 +353,6 b' class DOMWidget(Widget):'
         else:
             return None
 
-
     def set_css(self, *args, **kwargs):
         """Set one or more CSS properties of the widget.