diff --git a/includes/form.inc b/includes/form.inc
index b139e43f751..da1caa819f4 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -3311,6 +3311,8 @@ function form_process_container($element, &$form_state) {
  */
 function theme_container($variables) {
   $element = $variables['element'];
+  // Ensure #attributes is set.
+  $element += array('#attributes' => array());
 
   // Special handling for form elements.
   if (isset($element['#array_parents'])) {
diff --git a/modules/simpletest/tests/theme.test b/modules/simpletest/tests/theme.test
index 328afec44dd..f1a743e00c0 100644
--- a/modules/simpletest/tests/theme.test
+++ b/modules/simpletest/tests/theme.test
@@ -425,28 +425,100 @@ class ThemeFastTestCase extends DrupalWebTestCase {
 }
 
 /**
- * Unit tests for theme_html_tag().
+ * Tests the markup of core render element types passed to drupal_render().
  */
-class ThemeHtmlTag extends DrupalUnitTestCase {
+class RenderElementTypesTestCase extends DrupalWebTestCase {
   public static function getInfo() {
     return array(
-      'name' => 'Theme HTML Tag',
-      'description' => 'Tests theme_html_tag() built-in theme functions.',
+      'name' => 'Render element types',
+      'description' => 'Tests the markup of core render element types passed to drupal_render().',
       'group' => 'Theme',
     );
   }
 
   /**
-   * Test function theme_html_tag()
+   * Asserts that an array of elements is rendered properly.
+   *
+   * @param array $elements
+   *   An array of associative arrays describing render elements and their
+   *   expected markup. Each item in $elements must contain the following:
+   *   - 'name': This human readable description will be displayed on the test
+   *     results page.
+   *   - 'value': This is the render element to test.
+   *   - 'expected': This is the expected markup for the element in 'value'.
    */
-  function testThemeHtmlTag() {
-    // Test auto-closure meta tag generation
-    $tag['element'] = array('#tag' => 'meta', '#attributes' => array('name' => 'description', 'content' => 'Drupal test'));
-    $this->assertEqual('<meta name="description" content="Drupal test" />'."\n", theme_html_tag($tag), 'Test auto-closure meta tag generation.');
+  function assertElements($elements) {
+    foreach($elements as $element) {
+      $this->assertIdentical(drupal_render($element['value']), $element['expected'], '"' . $element['name'] . '" input rendered correctly by drupal_render().');
+    }
+  }
 
-    // Test title tag generation
-    $tag['element'] = array('#tag' => 'title', '#value' => 'title test');
-    $this->assertEqual('<title>title test</title>'."\n", theme_html_tag($tag), 'Test title tag generation.');
+  /**
+   * Tests system #type 'container'.
+   */
+  function testContainer() {
+    $elements = array(
+      // Basic container with no attributes.
+      array(
+        'name' => "#type 'container' with no HTML attributes",
+        'value' => array(
+          '#type' => 'container',
+          'child' => array(
+            '#markup' => 'foo',
+          ),
+        ),
+        'expected' => '<div>foo</div>',
+      ),
+      // Container with a class.
+      array(
+        'name' => "#type 'container' with a class HTML attribute",
+        'value' => array(
+          '#type' => 'container',
+          'child' => array(
+            '#markup' => 'foo',
+          ),
+          '#attributes' => array(
+            'class' => 'bar',
+          ),
+        ),
+        'expected' => '<div class="bar">foo</div>',
+      ),
+    );
+
+    $this->assertElements($elements);
+  }
+
+  /**
+   * Tests system #type 'html_tag'.
+   */
+  function testHtmlTag() {
+    $elements = array(
+      // Test auto-closure meta tag generation.
+      array(
+        'name' => "#type 'html_tag' auto-closure meta tag generation",
+        'value' => array(
+          '#type' => 'html_tag',
+          '#tag' => 'meta',
+          '#attributes' => array(
+            'name' => 'description',
+            'content' => 'Drupal test',
+          ),
+        ),
+        'expected' => '<meta name="description" content="Drupal test" />' . "\n",
+      ),
+      // Test title tag generation.
+      array(
+        'name' => "#type 'html_tag' title tag generation",
+        'value' => array(
+          '#type' => 'html_tag',
+          '#tag' => 'title',
+          '#value' => 'title test',
+        ),
+        'expected' => '<title>title test</title>' . "\n",
+      ),
+    );
+
+    $this->assertElements($elements);
   }
 }