diff --git a/core/lib/Drupal/Core/Asset/CssOptimizer.php b/core/lib/Drupal/Core/Asset/CssOptimizer.php
index 2cf07e7..2165b49 100644
--- a/core/lib/Drupal/Core/Asset/CssOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/CssOptimizer.php
@@ -96,17 +96,23 @@ public function loadFile($file, $optimize = NULL, $reset_basepath = TRUE) {
     if ($basepath && !file_uri_scheme($file)) {
       $file = $basepath . '/' . $file;
     }
+    // Store the parent base path to restore it later.
+    $parent_base_path = $basepath;
+    // Set the current base path to process possible child imports.
     $basepath = dirname($file);
 
     // Load the CSS stylesheet. We suppress errors because themes may specify
     // stylesheets in their .info.yml file that don't exist in the theme's path,
     // but are merely there to disable certain module CSS files.
+    $content = '';
     if ($contents = @file_get_contents($file)) {
       // Return the processed stylesheet.
-      return $this->processCss($contents, $_optimize);
+      $content = $this->processCss($contents, $_optimize);
     }
 
-    return '';
+    // Restore the parent base path as the file and its childen are processed.
+    $basepath = $parent_base_path;
+    return $content;
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php b/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php
index 41eb529..f37d716 100644
--- a/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php
@@ -169,7 +169,23 @@ function testOptimizeProvider() {
           'browsers' => array('IE' => TRUE, '!IE' => TRUE),
         ),
         '#home body { position: fixed; }',
-      )
+      ),
+      // Subfolder file. Tests:
+      // - CSS import path is properly interpreted.
+      5 => array(
+        array(
+          'group' => -100,
+          'every_page' => TRUE,
+          'type' => 'file',
+          'weight' => 0.013,
+          'media' => 'all',
+          'preprocess' => TRUE,
+          'data' => $path . 'css_subfolder/css_input_with_import.css',
+          'browsers' => array('IE' => TRUE, '!IE' => TRUE),
+          'basename' => 'css_input_with_import.css',
+        ),
+        str_replace('url(../images/icon.png)', 'url(' . file_create_url($path . 'images/icon.png') . ')', file_get_contents($path . 'css_subfolder/css_input_with_import.css.optimized.css')),
+      ),
     );
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_subfolder/css_input_with_import.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_subfolder/css_input_with_import.css
new file mode 100644
index 0000000..d90ecbc
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_subfolder/css_input_with_import.css
@@ -0,0 +1,29 @@
+
+
+@import "../import1.css";
+@import "../import2.css";
+
+body {
+  margin: 0;
+  padding: 0;
+  background: #edf5fa;
+  font: 76%/170% Verdana, sans-serif;
+  color: #494949;
+}
+
+.this .is .a .test {
+  font: 1em/100% Verdana, sans-serif;
+  color: #494949;
+}
+.this
+.is
+.a
+.test {
+font: 1em/100% Verdana, sans-serif;
+color: #494949;
+}
+
+textarea, select {
+  font: 1em/160% Verdana, sans-serif;
+  color: #494949;
+}
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_subfolder/css_input_with_import.css.optimized.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_subfolder/css_input_with_import.css.optimized.css
new file mode 100644
index 0000000..aba3b21
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/css_test_files/css_subfolder/css_input_with_import.css.optimized.css
@@ -0,0 +1,6 @@
+ul,select{font:1em/160% Verdana,sans-serif;color:#494949;}.ui-icon{background-image:url(../images/icon.png);}
+p,select{font:1em/160% Verdana,sans-serif;color:#494949;}
+body{margin:0;padding:0;background:#edf5fa;font:76%/170% Verdana,sans-serif;color:#494949;}.this .is .a .test{font:1em/100% Verdana,sans-serif;color:#494949;}.this
+.is
+.a
+.test{font:1em/100% Verdana,sans-serif;color:#494949;}textarea,select{font:1em/160% Verdana,sans-serif;color:#494949;}