diff --git a/core/modules/system/src/Tests/Installer/StandardInstallerTest.php b/core/modules/system/src/Tests/Installer/StandardInstallerTest.php
index 184e07b..fdbbd99 100644
--- a/core/modules/system/src/Tests/Installer/StandardInstallerTest.php
+++ b/core/modules/system/src/Tests/Installer/StandardInstallerTest.php
@@ -41,6 +41,17 @@ protected function setUpSite() {
   }
 
   /**
+   * {@inheritdoc}
+   */
+  protected function curlExec($curl_options, $redirect = FALSE) {
+    // Ensure that we see the classy progress css on the page.
+    if (strpos($curl_options[CURLOPT_URL], '&id=1&op=do_nojs') !== FALSE) {
+      $this->assertRaw('themes/classy/css/components/progress.css');
+    }
+    return parent::curlExec($curl_options, $redirect);
+  }
+
+  /**
    * Ensures that the exported standard configuration is up to date.
    */
   public function testStandardConfig() {