diff -u b/includes/common.inc b/includes/common.inc
--- b/includes/common.inc
+++ b/includes/common.inc
@@ -2944,7 +2944,7 @@
 }
 
 /**
- * Adds a LINK tag with a distinct attribute to the page's HEAD.
+ * Adds a LINK tag with distinct attributes to the page's HEAD.
  *
  * This function can be called as long the HTML header hasn't been sent, which
  * on normal pages is up through the preprocess step of theme('html'). Adding
@@ -2952,8 +2952,7 @@
  * 'hreflang' attributes.
  *
  * @param $attributes
- *   Associative array of element attributes including 'rel', 'href' and
- *   'hreflang'.
+ *   Associative array of element attributes including 'href' and 'rel'.
  * @param $header
  *   Optional flag to determine if a HTTP 'Link:' header should be sent.
  */
@@ -2971,7 +2970,7 @@
     $element['#attached']['drupal_add_http_header'][] = array('Link',  $href . drupal_http_header_attributes($attributes), TRUE);
   }
 
-  drupal_add_html_head($element, 'drupal_add_html_head_link:' . (isset($attributes['hreflang']) ? $attributes['hreflang'] . ':' : '') . $attributes['rel'] . ':' . $href);
+  drupal_add_html_head($element, 'drupal_add_html_head_link:' . $attributes['rel'] . ':' . (isset($attributes['hreflang']) ? "{$attributes['hreflang']}:" : '') . $href);
 }
 
 /**
only in patch2:
unchanged:
--- a/modules/simpletest/tests/common.test
+++ b/modules/simpletest/tests/common.test
@@ -3297,3 +3297,45 @@ class BlockInterestCohortTest extends DrupalWebTestCase {
   }
 
 }
+
+/**
+ * Test for drupal_add_html_head_link().
+ */
+class DrupalAddHtmlHeadLinkTest extends DrupalWebTestCase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getInfo() {
+    return array(
+      'name' => 'Add HTML head link',
+      'description' => 'Test for drupal_add_html_head_link().',
+      'group' => 'System',
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  function setUp() {
+    parent::setUp('common_test');
+  }
+
+  /**
+   * Tests drupal_add_html_head_link().
+   */
+  function testDrupalAddHtmlHeadLink() {
+    $this->drupalGet('common-test/html_head_link');
+    $expected_link_header = implode(',', array(
+      '</foo?bar=&lt;baz&gt;&amp;baz=false>; rel="alternate"',
+      '</foo/bar>; hreflang="nl"; rel="alternate"',
+      '</foo/bar>; hreflang="de"; rel="alternate"',
+    ));
+    $this->assertEqual($this->drupalGetHeader('Link'), $expected_link_header);
+
+    // Check that duplicate alternate URLs with different hreflangs are allowed.
+    $test_link = $this->xpath('//head/link[@rel="alternate"][@href="/foo/bar"]');
+    $this->assertEqual(count($test_link), 2, 'Duplicate alternate URLs are allowed.');
+  }
+
+}
only in patch2:
unchanged:
--- a/modules/simpletest/tests/common_test.module
+++ b/modules/simpletest/tests/common_test.module
@@ -64,6 +64,12 @@ function common_test_menu() {
     'access arguments' => array('access content'),
     'type' => MENU_CALLBACK,
   );
+  $items['common-test/html_head_link'] = array(
+    'title' => 'Test HTML head link',
+    'page callback' => 'common_test_html_head_link',
+    'access arguments' => array('access content'),
+    'type' => MENU_CALLBACK,
+  );
   return $items;
 }
 
@@ -314,3 +320,28 @@ function existing_permissions_policy_header() {
   drupal_add_http_header('Permissions-Policy', 'geolocation=()');
   print __FUNCTION__;
 }
+
+/**
+ * Page callback.
+ */
+function common_test_html_head_link() {
+  drupal_add_html_head_link(array(
+    'href' => '/foo?bar=<baz>&baz=false',
+    'rel' => 'alternate',
+  ), TRUE);
+  drupal_add_html_head_link(array(
+    'href' => '/not-added-to-http-headers',
+    'rel' => 'alternate',
+  ), FALSE);
+  drupal_add_html_head_link(array(
+    'href' => '/foo/bar',
+    'hreflang' => 'nl',
+    'rel' => 'alternate',
+  ), TRUE);
+  drupal_add_html_head_link(array(
+    'href' => '/foo/bar',
+    'hreflang' => 'de',
+    'rel' => 'alternate',
+  ), TRUE);
+  return '';
+}