diff -U7 b/core/modules/image/image.module b/core/modules/image/image.module
--- b/core/modules/image/image.module
+++ b/core/modules/image/image.module
@@ -146,14 +146,26 @@
 /**
  * Implements hook_theme().
  */
 function image_theme() {
   return array(
     // Theme functions in image.module.
     'image_style' => array(
+      // HTML 4 and XHTML 1.0 always require an alt attribute. The HTML 5 draft
+      // allows the alt attribute to be omitted in some cases. Therefore,
+      // default the alt attribute to an empty string, but allow code calling
+      // theme('image_style') to pass explicit NULL for it to be omitted.
+      //  Usually, neither omission nor an empty string satisfies accessibility
+      // requirements, so it is strongly encouraged for code calling
+      // theme('image_style') to pass a meaningful value for the alt variable.
+      // - http://www.w3.org/TR/REC-html40/struct/objects.html#h-13.8
+      // - http://www.w3.org/TR/xhtml1/dtds.html
+      // - http://dev.w3.org/html5/spec/Overview.html#alt
+      // The title attribute is optional in all cases, so it is omitted by
+      // default.
       'variables' => array(
         'style_name' => NULL,
         'uri' => NULL,
         'width' => NULL,
         'height' => NULL,
         'alt' => '',
         'title' => NULL,
@@ -358,15 +370,24 @@
  *   - uri: The path of the image file relative to the Drupal files directory.
  *     This function does not work with images outside the files directory nor
  *     with remotely hosted images. This should be in a format such as
  *     'images/image.jpg', or using a stream wrapper such as
  *     'public://images/image.jpg'.
  *   - width: The width of the source image (if known).
  *   - height: The height of the source image (if known).
- *   - alt: The alternative text for text-based browsers.
+ *   - alt: The alternative text for text-based browsers. HTML 4 and XHTML 1.0
+ *     always require an alt attribute. The HTML 5 draft allows the alt
+ *     attribute to be omitted in some cases. Therefore, this variable defaults
+ *     to an empty string, but can be set to NULL for the attribute to be
+ *     omitted. Usually, neither omission nor an empty string satisfies
+ *     accessibility requirements, so it is strongly encouraged for code calling
+ *     theme('image_style') to pass a meaningful value for this variable.
+ *     - http://www.w3.org/TR/REC-html40/struct/objects.html#h-13.8
+ *     - http://www.w3.org/TR/xhtml1/dtds.html
+ *     - http://dev.w3.org/html5/spec/Overview.html#alt
  *   - title: The title text is displayed when the image is hovered in some
  *     popular browsers.
  *   - attributes: Associative array of attributes to be placed in the img tag.
  *
  * @ingroup themeable
  */
 function theme_image_style($variables) {