diff --git a/core/includes/common.inc b/core/includes/common.inc
index a392a5a..258f5bf 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -2385,17 +2385,23 @@ function drupal_load_stylesheet($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($file)) {
     // Return the processed stylesheet.
-    return drupal_load_stylesheet_content($contents, $_optimize);
+    $content = drupal_load_stylesheet_content($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/modules/simpletest/files/css_test_files/css_input_with_import.css.unoptimized.css b/core/modules/simpletest/files/css_test_files/css_input_with_import.css.unoptimized.css
index 4c905f5..19323c1 100644
--- a/core/modules/simpletest/files/css_test_files/css_input_with_import.css.unoptimized.css
+++ b/core/modules/simpletest/files/css_test_files/css_input_with_import.css.unoptimized.css
@@ -1,6 +1,16 @@
 
 
 
+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 {
diff --git a/core/modules/simpletest/files/css_test_files/css_subfolder/css_input_with_import.css b/core/modules/simpletest/files/css_test_files/css_subfolder/css_input_with_import.css
new file mode 100644
index 0000000..5ed72ee
--- /dev/null
+++ b/core/modules/simpletest/files/css_test_files/css_subfolder/css_input_with_import.css
@@ -0,0 +1,30 @@
+
+
+@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/modules/simpletest/files/css_test_files/css_subfolder/css_input_with_import.css.optimized.css b/core/modules/simpletest/files/css_test_files/css_subfolder/css_input_with_import.css.optimized.css
new file mode 100644
index 0000000..aba3b21
--- /dev/null
+++ b/core/modules/simpletest/files/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;}
diff --git a/core/modules/simpletest/files/css_test_files/css_subfolder/css_input_with_import.css.unoptimized.css b/core/modules/simpletest/files/css_test_files/css_subfolder/css_input_with_import.css.unoptimized.css
new file mode 100644
index 0000000..50fb4a8
--- /dev/null
+++ b/core/modules/simpletest/files/css_test_files/css_subfolder/css_input_with_import.css.unoptimized.css
@@ -0,0 +1,40 @@
+
+
+
+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;
+}
+
diff --git a/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsUnitTest.php b/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsUnitTest.php
index 5ab3c0a..d41800b 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsUnitTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Common/CascadingStylesheetsUnitTest.php
@@ -38,26 +38,30 @@ function testLoadCssBasic() {
     $testfiles = array(
       'css_input_without_import.css',
       'css_input_with_import.css',
+      'css_subfolder/css_input_with_import.css',
       'comment_hacks.css'
     );
     $path = drupal_get_path('module', 'simpletest') . '/files/css_test_files';
     foreach ($testfiles as $file) {
-      $expected = file_get_contents("$path/$file.unoptimized.css");
-      $unoptimized_output = drupal_load_stylesheet("$path/$file.unoptimized.css", FALSE);
+      $file_path = $path . '/' . $file;
+      $file_url = $GLOBALS['base_url'] . '/' . $file_path;
+
+      $expected = file_get_contents($file_path . '.unoptimized.css');
+      $unoptimized_output = drupal_load_stylesheet($file_path, FALSE);
       $this->assertEqual($unoptimized_output, $expected, format_string('Unoptimized CSS file has expected contents (@file)', array('@file' => $file)));
 
-      $expected = file_get_contents("$path/$file.optimized.css");
-      $optimized_output = drupal_load_stylesheet("$path/$file", TRUE);
+      $expected = file_get_contents($file_path . '.optimized.css');
+      $optimized_output = drupal_load_stylesheet($file_path, TRUE);
       $this->assertEqual($optimized_output, $expected, format_string('Optimized CSS file has expected contents (@file)', array('@file' => $file)));
 
       // Repeat the tests by accessing the stylesheets by URL.
-      $expected = file_get_contents("$path/$file.unoptimized.css");
-      $unoptimized_output_url = drupal_load_stylesheet($GLOBALS['base_url'] . "/$path/$file.unoptimized.css", FALSE);
-      $this->assertEqual($unoptimized_output, $expected, format_string('Unoptimized CSS file (loaded from an URL) has expected contents (@file)', array('@file' => $file)));
+      $expected = file_get_contents($file_path . '.unoptimized.css');
+      $unoptimized_output_url = drupal_load_stylesheet($file_url, FALSE);
+      $this->assertEqual($unoptimized_output_url, $expected, format_string('Unoptimized CSS file (loaded from an URL) has expected contents (@file)', array('@file' => $file)));
 
-      $expected = file_get_contents("$path/$file.optimized.css");
-      $optimized_output = drupal_load_stylesheet($GLOBALS['base_url'] . "/$path/$file", TRUE);
-      $this->assertEqual($optimized_output, $expected, format_string('Optimized CSS file (loaded from an URL) has expected contents (@file)', array('@file' => $file)));
+      $expected = file_get_contents($file_path . '.optimized.css');
+      $optimized_output_url = drupal_load_stylesheet($file_url, TRUE);
+      $this->assertEqual($optimized_output_url, $expected, format_string('Optimized CSS file (loaded from an URL) has expected contents (@file)', array('@file' => $file)));
     }
   }
 }