diff --git a/wp-includes/customize/class-wp-customize-custom-css-setting.php b/wp-includes/customize/class-wp-customize-custom-css-setting.php
index b0fd89540b..0e6132f339 100644
--- a/wp-includes/customize/class-wp-customize-custom-css-setting.php
+++ b/wp-includes/customize/class-wp-customize-custom-css-setting.php
@@ -145,18 +145,22 @@ final class WP_Customize_Custom_CSS_Setting extends WP_Customize_Setting {
 	}
 
 	/**
-	 * Validate CSS.
+	 * Validate a received value for being valid CSS.
 	 *
 	 * Checks for imbalanced braces, brackets, and comments.
 	 * Notifications are rendered when the customizer state is saved.
 	 *
 	 * @since 4.7.0
 	 * @since 4.9.0 Checking for balanced characters has been moved client-side via linting in code editor.
+	 * @since 5.9.0 Renamed `$css` to `$value` for PHP 8 named parameter support.
 	 *
-	 * @param string $css The input string.
+	 * @param string $value CSS to validate.
 	 * @return true|WP_Error True if the input was validated, otherwise WP_Error.
 	 */
-	public function validate( $css ) {
+	public function validate( $value ) {
+		// Restores the more descriptive, specific name for use within this method.
+		$css = $value;
+
 		$validity = new WP_Error();
 
 		if ( preg_match( '#</?\w+#', $css ) ) {
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 860fde97ec..8de4d037d2 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51784';
+$wp_version = '5.9-alpha-51785';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.