diff --git a/core/lib/Drupal/Core/Asset/CssOptimizer.php b/core/lib/Drupal/Core/Asset/CssOptimizer.php
index 4254a15..dfe2665 100644
--- a/core/lib/Drupal/Core/Asset/CssOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/CssOptimizer.php
@@ -53,7 +53,13 @@ public function clean($contents) {
   }
 
   /**
-   * Build aggregate CSS file.
+   * Processes CSS file and add base URLs to any relative resource paths.
+   *
+   * @param array $css_asset
+   *   A CSS asset.
+   *
+   * @return string
+   *   The asset's cleaned/optimized contents.
    */
   protected function processFile($css_asset) {
     $contents = $this->loadFile($css_asset['data'], TRUE);