gistfile1.txt

diff --git a/core/lib/Drupal/Core/Asset/CssOptimizer.php b/core/lib/Drupal/Core/Asset/CssOptimizer.php
index 775603b633..93f2220c42 100644
--- a/core/lib/Drupal/Core/Asset/CssOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/CssOptimizer.php
@@ -189,8 +189,8 @@ protected function processCss($contents, $optimize = FALSE) {
 
     if ($optimize) {
       // Perform some safe CSS optimizations.
-      // Regexp to match comment blocks.
-      $comment = '/\*[^*]*\*+(?:[^/*][^*]*\*+)*/';
+      // Regexp to match comment blocks, except for "/*!*/" (so that "/*!*/ /*!*/" is preserved).
+      $comment = '/\*(?!!\*/)[^*]*\*+(?:[^/*][^*]*\*+)*/';
       // Regexp to match double quoted strings.
       $double_quot = '"[^"\\\\]*(?:\\\\.[^"\\\\]*)*"';
       // Regexp to match single quoted strings.