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
@@ -621,7 +621,9 @@
  * CSS identifier double underscores allowance:
  *
  * To allow CSS identifiers to contain double underscores (.example__selector)
- * for BEM-style naming standards, uncomment the line below.
+ * for Drupal's BEM-style naming standards, uncomment the line below.
+ * Note that if you change this value in existing sites, existing page styles
+ * may be broken.
  *
  * @see drupal_clean_css_identifier()
  */