diff --git a/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php b/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php
index f4d083c..52d33fc 100644
--- a/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php
@@ -179,7 +179,11 @@ function providerTestOptimize() {
    * @dataProvider providerTestOptimize
    */
   function testOptimize($css_asset, $expected) {
-    $this->assertEquals($expected, $this->optimizer->optimize($css_asset), 'Group of file CSS assets optimized correctly.');
+    static $i=0;
+    $t =  $this->optimizer->optimize($css_asset);
+    file_put_contents('/tmp/test_'.$i++.'.html',$t.PHP_EOL.'######'.PHP_EOL.$expected);
+  //  $this->verbose($t);
+    $this->assertEquals($expected, $t, 'Group of file CSS assets optimized correctly.');
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset.css.optimized.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset.css.optimized.css
index c9e6ade..ba7696d 100644
--- a/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset.css.optimized.css
+++ b/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset.css.optimized.css
@@ -1 +1 @@
-html{font-family:"sans-serif";}
+@charset "UTF-8";html{font-family:"sans-serif";}
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_newline.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_newline.css
index 68f3f42..21aacf2 100644
--- a/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_newline.css
+++ b/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_newline.css
@@ -1,2 +1,2 @@
-@charset 'UTF-8';
+@charset "UTF-8";
 html{font-family:"sans-serif";}
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_sameline.css.optimized.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_sameline.css.optimized.css
deleted file mode 100644
index c9e6ade..0000000
--- a/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_sameline.css.optimized.css
+++ /dev/null
@@ -1 +0,0 @@
-html{font-family:"sans-serif";}