diff --git a/includes/common.inc b/includes/common.inc
index 00b2e71..a203dfc 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -3626,10 +3626,20 @@ function drupal_delete_file_if_stale($uri) {
  * @return
  *   The cleaned identifier.
  */
-function drupal_clean_css_identifier($identifier, $filter = array(' ' => '-', '_' => '-', '/' => '-', '[' => '-', ']' => '')) {
-  // By default, we filter using Drupal's coding standards.
+function drupal_clean_css_identifier($identifier, $filter = array(' ' => '-')) {
+  // replace whitespaces
   $identifier = strtr($identifier, $filter);
 
+  // CSS class identifiers must not start with
+  // - a digit
+  // - two hyphens
+  // - hyphen followed by a digit
+  $prev = '';
+  while ($prev != $identifier) {
+    $prev = $identifier;
+    $identifier = preg_replace(array('/^\d*/u', '/^\x{002D}*(?=\x{002D})/u', '/^\x{002D}\d/u'),'', $identifier);
+  }
+
   // Valid characters in a CSS identifier are:
   // - the hyphen (U+002D)
   // - a-z (U+0030 - U+0039)
@@ -3638,6 +3648,8 @@ function drupal_clean_css_identifier($identifier, $filter = array(' ' => '-', '_
   // - 0-9 (U+0061 - U+007A)
   // - ISO 10646 characters U+00A1 and higher
   // We strip out any character not in the above list.
+  // as opposed to the spec we do not allow U+00A0
+  // as opposed to the spec we only allow code points up to U+FFFF
   $identifier = preg_replace('/[^\x{002D}\x{0030}-\x{0039}\x{0041}-\x{005A}\x{005F}\x{0061}-\x{007A}\x{00A1}-\x{FFFF}]/u', '', $identifier);
 
   return $identifier;
diff --git a/modules/simpletest/tests/common.test b/modules/simpletest/tests/common.test
index 4664f04..d20bf82 100644
--- a/modules/simpletest/tests/common.test
+++ b/modules/simpletest/tests/common.test
@@ -812,6 +812,22 @@ class DrupalHTMLIdentifierTestCase extends DrupalUnitTestCase {
 
     // Verify that invalid characters (including non-breaking space) are stripped from the identifier.
     $this->assertIdentical(drupal_clean_css_identifier('invalid !"#$%&\'()*+,./:;<=>?@[\\]^`{|}~ identifier', array()), 'invalididentifier', t('Strip invalid characters.'));
+
+    $identifier = array(
+      '-foo'      => '----foo',
+      '-bar'      => '-bar',
+      'foo'       => 'foo',
+      'bar'       => '1bar',
+      'foobar'    => '1111foobar',
+      '-foofoo'   => '1111---foofoo',
+      '-barbar'   => '-1--barbar',
+      '-_foo-bar' => '---11---_foo bar',
+      'foo_bar'   => 'foo_bar',
+      '☺'        => '☺'
+    );
+    foreach ($identifier as $expected => $given) {
+      $this->assertIdentical(drupal_clean_css_identifier($given), $expected, t('Strip invalid trailing characters.'));
+    }
   }
 
   /**