diff -u b/includes/common.inc b/includes/common.inc
--- b/includes/common.inc
+++ b/includes/common.inc
@@ -3873,14 +3873,6 @@
  *   The cleaned identifier.
  */
 function drupal_clean_css_identifier($identifier, $filter = array(' ' => '-', '_' => '-', '/' => '-', '[' => '-', ']' => '')) {
-  global $conf;
-
-  // Preserve BEM-style double-underscores depending on custom setting.
-  // Avoids a variable_get function call to improve performance.
-  if (!empty($conf['preserve_css_double_underscores'])) {
-    $filter['__'] = '__';
-  }
-
   // By default, we filter using Drupal's coding standards.
   $identifier = strtr($identifier, $filter);
 
@@ -3901,6 +3893,21 @@
  *   The cleaned identifier.
  */
 function drupal_clean_css_identifier($identifier, $filter = array(' ' => '-', '_' => '-', '/' => '-', '[' => '-', ']' => '')) {
+  // Use the advanced drupal_static() pattern, since this is called very often.
+  static $drupal_static_fast;
+  if (!isset($drupal_static_fast)) {
+    $drupal_static_fast['allow_css_double_underscores'] = &drupal_static(__FUNCTION__ . ':allow_css_double_underscores');
+  }
+  $allow_css_double_underscores = &$drupal_static_fast['allow_css_double_underscores'];
+  if (!isset($allow_css_double_underscores)) {
+    $allow_css_double_underscores = variable_get('allow_css_double_underscores');
+  }
+
+  // Preserve BEM-style double-underscores depending on custom setting.
+  if ($allow_css_double_underscores) {
+    $filter['__'] = '__';
+  }
+
   // By default, we filter using Drupal's coding standards.
   $identifier = strtr($identifier, $filter);
 
diff -u b/modules/simpletest/tests/common.test b/modules/simpletest/tests/common.test
--- b/modules/simpletest/tests/common.test
+++ b/modules/simpletest/tests/common.test
@@ -948,20 +948,20 @@
     // Verify that invalid characters (including non-breaking space) are stripped from the identifier.
     $this->assertIdentical(drupal_clean_css_identifier('invalid !"#$%&\'()*+,./:;<=>?@[\\]^`{|}~ identifier', array()), 'invalididentifier', 'Strip invalid characters.');
 
-    // Verify that double underscores are replaced in the identifier.
+    // Verify that double underscores are replaced in the identifier by default.
     $identifier = 'css__identifier__with__double__underscores';
     $expected = 'css--identifier--with--double--underscores';
-    $this->assertIdentical(drupal_clean_css_identifier($identifier), $expected, 'Verify double underscores are replaced with double hyphens.');
+    $this->assertIdentical(drupal_clean_css_identifier($identifier), $expected, 'Verify double underscores are replaced with double hyphens by default.');
 
-    // Mock a variable_set of preserve_css_double_underscores to TRUE.
-    $original = $GLOBALS['conf']['preserve_css_double_underscores'];
-    $GLOBALS['conf']['preserve_css_double_underscores'] = TRUE;
+    // Mock a variable_set of allow_css_double_underscores to TRUE.
+    $original = isset($GLOBALS['conf']['allow_css_double_underscores']) ? $GLOBALS['conf']['allow_css_double_underscores'] : NULL;
+    $GLOBALS['conf']['allow_css_double_underscores'] = TRUE;
 
     // Now, verify that double underscores are preserved in the identifier.
     $this->assertIdentical(drupal_clean_css_identifier($identifier), $identifier, 'Verify double underscores are preserved.');
 
     // Revert the overridden value above.
-    $GLOBALS['conf']['preserve_css_double_underscores'] = $original;
+    $GLOBALS['conf']['allow_css_double_underscores'] = $original;
   }
 
   /**
diff -u b/sites/default/default.settings.php b/sites/default/default.settings.php
--- b/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -612,7 +612,8 @@
- * Preserve CSS double underscores:
+ * CSS identifier double underscores allowance:
  *
- * To allow CSS identifiers containing double underscores, e.g., in a BEM-style
- * selector (.example__selector), uncomment the line below.
+ * To allow CSS identifiers to contain double underscores (.example__selector)
+ * for BEM-style naming standards, uncomment the line below.
+ *
+ * @see drupal_clean_css_identifier()
  */
-# $conf['preserve_css_double_underscores'] = TRUE;
-
+# $conf['allow_css_double_underscores'] = TRUE;