diff --git a/packages/ui/attrs2.js b/packages/ui/attrs2.js
new file mode 100644
index 0000000000..450292e1fa
--- /dev/null
+++ b/packages/ui/attrs2.js
@@ -0,0 +1,86 @@
+
+// An AttributeHandler object is responsible for updating a particular attribute
+// of a particular element.  AttributeHandler subclasses implement
+// browser-specific logic for dealing with particular attributes across
+// different browsers.
+//
+// To define a new type of AttributeHandler, use
+// `var FooHandler = AttributeHandler.extend({ update: function ... })`
+// where the `update` function takes arguments `(element, oldValue, value)`.
+// The `element` argument is always the same between calls to `update` on
+// the same instance.  `oldValue` and `value` are each either `null` or
+// a Unicode string of the type that might be passed to the value argument
+// of `setAttribute` (i.e. not an HTML string with character references).
+// When an AttributeHandler is installed, an initial call to `update` is
+// always made with `oldValue = null`.  The `update` method can access
+// `this.name` if the AttributeHandler class is a generic one that applies
+// to multiple attribute names.
+//
+// AttributeHandlers can store custom properties on `this`, as long as they
+// don't use the names `element`, `name`, `value`, and `oldValue`.
+//
+// AttributeHandlers can't influence how attributes appear in rendered HTML,
+// only how they are updated after materialization as DOM.
+
+AttributeHandler2 = function (name, value) {
+  this.name = name;
+  this.value = value;
+};
+
+_extend(AttributeHandler2.prototype, {
+  update: function (element, oldValue, value) {
+    if (value === null) {
+      if (oldValue !== null)
+        element.removeAttribute(this.name);
+    } else {
+      element.setAttribute(this.name, this.value);
+    }
+  }
+});
+
+AttributeHandler2.extend = function (options) {
+  var curType = this;
+  var subType = function AttributeHandlerSubtype(/*arguments*/) {
+    AttributeHandler2.apply(this, arguments);
+  };
+  subType.prototype = new curType;
+  subType.extend = curType.extend;
+  if (options)
+    _.extend(subType.prototype, options);
+  return subType;
+};
+
+// Value of a ClassHandler is either a string or an array.
+var ClassHandler = AttributeHandler2.extend({
+  update: function (element, oldValue, value) {
+    var oldClasses = oldValue ? _.compact(oldValue.split(' ')) : [];
+    var newClasses = value ? _.compact(value.split(' ')) : [];
+
+    // the current classes on the element, which we will mutate.
+    var classes = _.compact(element.className.split(' '));
+
+    // optimize this later (to be asymptotically faster) if necessary
+    _.each(oldClasses, function (c) {
+      if (_.indexOf(newClasses, c) < 0)
+        classes = _.without(classes, c);
+    });
+    _.each(newClasses, function (c) {
+      if (_.indexOf(oldClasses, c) < 0 &&
+          _.indexOf(classes, c) < 0)
+        classes.push(c);
+    });
+
+    element.className = classes.join(' ');
+  }
+});
+
+// XXX make it possible for users to register attribute handlers!
+makeAttributeHandler2 = function (name, value) {
+  // XXX will need one for 'style' on IE, though modern browsers
+  // seem to handle setAttribute ok.
+  if (name === 'class') {
+    return new ClassHandler(name, value);
+  } else {
+    return new AttributeHandler2(name, value);
+  }
+};
diff --git a/packages/ui/exceptions.js b/packages/ui/exceptions.js
new file mode 100644
index 0000000000..b72fec2160
--- /dev/null
+++ b/packages/ui/exceptions.js
@@ -0,0 +1,34 @@
+
+var debugFunc;
+
+// Meteor UI calls into user code in many places, and it's nice to catch exceptions
+// propagated from user code immediately so that the whole system doesn't just
+// break.  Catching exceptions is easy; reporting them is hard.  This helper
+// reports exceptions.
+//
+// Usage:
+//
+// ```
+// try {
+//   // ... someStuff ...
+// } catch (e) {
+//   reportUIException(e);
+// }
+// ```
+//
+// An optional second argument overrides the default message.
+
+reportUIException = function (e, msg) {
+  if (! debugFunc)
+    // adapted from Deps
+    debugFunc = function () {
+      return (typeof Meteor !== "undefined" ? Meteor._debug :
+              ((typeof console !== "undefined") && console.log ? console.log :
+               function () {}));
+    };
+
+  // In Chrome, `e.stack` is a multiline string that starts with the message
+  // and contains a stack trace.  Furthermore, `console.log` makes it clickable.
+  // `console.log` supplies the space between the two arguments.
+  debugFunc(msg || 'Exception in Meteor UI:', e.stack || e.message);
+};
diff --git a/packages/ui/package.js b/packages/ui/package.js
index 4110d1444c..b77940e4b9 100644
--- a/packages/ui/package.js
+++ b/packages/ui/package.js
@@ -13,13 +13,14 @@ Package.on_use(function (api) {
   api.use('minimongo');  // for idStringify
   api.use('observe-sequence');
 
-  api.add_files(['base.js']);
+  api.add_files(['exceptions.js', 'base.js']);
 
   api.add_files(['dombackend.js',
                  'dombackend2.js',
                  'domrange.js'], 'client');
 
   api.add_files(['attrs.js',
+                 'attrs2.js',
                  'render.js',
                  'render2.js',
                  'components.js',
diff --git a/packages/ui/render2.js b/packages/ui/render2.js
index 8eb68500b1..e652098bbd 100644
--- a/packages/ui/render2.js
+++ b/packages/ui/render2.js
@@ -106,6 +106,46 @@ var insert = function (nodeOrRange, parent, before) {
   }
 };
 
+// Update attributes on `elem` to the dictionary `attrs`, using the
+// dictionary of existing `handlers` if provided.
+//
+// Values in the `attrs` dictionary are in pseudo-DOM form -- a string,
+// CharRef, or array of strings and CharRefs -- but they are passed to
+// the AttributeHandler in string form.
+var updateAttributes = function(elem, attrs, handlers) {
+  if (handlers) {
+    for (var k in handlers) {
+      if (! attrs.hasOwnProperty(k)) {
+        // remove old attributes (and handlers)
+        var handler = handlers[k];
+        var oldValue = handler.value;
+        handler.value = null;
+        handler.update(elem, oldValue, null);
+        delete handlers[k];
+      }
+    }
+  }
+
+  for (var k in attrs) {
+    var handler;
+    var oldValue;
+    var value = attributeValueToString(attrs[k]);
+    if ((! handlers) || (! handlers.hasOwnProperty(k))) {
+      // make new handler
+      checkAttributeName(k);
+      handler = makeAttributeHandler2(k, value);
+      if (handlers)
+        handlers[k] = handler;
+      oldValue = null;
+    } else {
+      handler = handlers[k];
+      oldValue = handler.value;
+    }
+    handler.value = value;
+    handler.update(elem, oldValue, value);
+  }
+};
+
 // Convert the pseudoDOM `node` into reactive DOM nodes and insert them
 // into the element or DomRange `parent`, before the node or id `before`.
 var materialize = function (node, parent, before) {
@@ -129,13 +169,23 @@ var materialize = function (node, parent, before) {
         var elem = document.createElement(node.tagName);
         if (node.attrs) {
           var attrs = node.attrs;
-          // XXX make attributes reactive!
-          if (typeof attrs === 'function')
-            attrs = attrs();
-          _.each(attrs, function (v, k) {
-            checkAttributeName(k);
-            elem.setAttribute(k, attributeValueToString(v));
-          });
+          if (typeof attrs === 'function') {
+            var attrUpdater = Deps.autorun(function (c) {
+              if (! c.handlers)
+                c.handlers = {};
+
+              try {
+                updateAttributes(elem, attrs(), c.handlers);
+              } catch (e) {
+                reportUIException(e);
+              }
+            });
+            UI.DomBackend2.onRemoveElement(elem, function () {
+              attrUpdater.stop();
+            });
+          } else {
+            updateAttributes(elem, attrs);
+          }
         }
         _.each(node, function (child) {
           materialize(child, elem);