- Patch #555830 by effulgentsia: better code comments.

includes/common.inc

@@ -5390,6 +5390,18 @@ function drupal_common_theme() {
       'variables' => array('links' => NULL, 'attributes' => array('class' => array('links')), 'heading' => array()),
     ),
     'image' => 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') 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') 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('path' => NULL, 'alt' => '', 'title' => NULL, 'attributes' => array(), 'getsize' => TRUE),
     ),
     'breadcrumb' => array(

includes/theme.inc

@@ -1470,7 +1470,16 @@ function theme_links($variables) {
  *   An associative array containing:
  *   - path: Either the path of the image file (relative to base_path()) or a
  *     full URL.
- *   - 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') 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.