--- a/markup/builder.py
+++ b/markup/builder.py
@@ -11,7 +11,7 @@
 # individuals. For the exact contribution history, see the revision
 # history and logs, available at http://projects.edgewall.com/trac/.
 
-from markup.core import Attributes, QName, Stream
+from markup.core import Attributes, Namespace, QName, Stream
 
 __all__ = ['Fragment', 'Element', 'tag']
 
@@ -171,8 +171,16 @@
 
 class ElementFactory(object):
 
-    def __getattribute__(self, name):
-        return Element(name.lower())
+    def __init__(self, namespace=None):
+        if not isinstance(namespace, Namespace):
+            namespace = Namespace(namespace)
+        self.namespace = namespace
+
+    def __getitem__(self, namespace):
+        return ElementFactory(namespace)
+
+    def __getattr__(self, name):
+        return Element(self.namespace and self.namespace[name] or name)
 
 
 tag = ElementFactory()

--- a/markup/output.py
+++ b/markup/output.py
@@ -103,9 +103,12 @@
                 tag = data
                 tagname = tag.localname
                 if tag.namespace:
-                    prefix = ns_mapping[tag.namespace]
-                    if prefix:
-                        tagname = prefix + ':' + tag.localname
+                    try:
+                        prefix = ns_mapping[tag.namespace]
+                        if prefix:
+                            tagname = prefix + ':' + tag.localname
+                    except KeyError:
+                        pass
                 yield Markup('</%s>' % tagname)
 
             elif kind is Stream.TEXT: