diff --git a/IPython/html/widgets/widget.py b/IPython/html/widgets/widget.py
index 1618565..974f248 100644
--- a/IPython/html/widgets/widget.py
+++ b/IPython/html/widgets/widget.py
@@ -427,7 +427,8 @@ class DOMWidget(Widget):
     
     width = CUnicode(sync=True)
     height = CUnicode(sync=True)
-    padding = CUnicode(2.5, sync=True)
+    # A default padding of 2.5 px makes the widgets look nice when displayed inline.
+    padding = CUnicode("2.5px", sync=True)
     margin = CUnicode(sync=True)
 
     color = Unicode(sync=True)