diff --git a/wp-includes/customize/class-wp-customize-background-image-setting.php b/wp-includes/customize/class-wp-customize-background-image-setting.php
index 96b69a022d..b8b1a06a9d 100644
--- a/wp-includes/customize/class-wp-customize-background-image-setting.php
+++ b/wp-includes/customize/class-wp-customize-background-image-setting.php
@@ -20,7 +20,7 @@ final class WP_Customize_Background_Image_Setting extends WP_Customize_Setting {
 	/**
 	 * @since 3.4.0
 	 *
-	 * @param mixed $value
+	 * @param mixed $value The value to update. Not used.
 	 */
 	public function update( $value ) {
 		remove_theme_mod( 'background_image_thumb' );
diff --git a/wp-includes/customize/class-wp-customize-custom-css-setting.php b/wp-includes/customize/class-wp-customize-custom-css-setting.php
index 39750f864c..b0fd89540b 100644
--- a/wp-includes/customize/class-wp-customize-custom-css-setting.php
+++ b/wp-includes/customize/class-wp-customize-custom-css-setting.php
@@ -173,11 +173,15 @@ final class WP_Customize_Custom_CSS_Setting extends WP_Customize_Setting {
 	 * Store the CSS setting value in the custom_css custom post type for the stylesheet.
 	 *
 	 * @since 4.7.0
+	 * @since 5.9.0 Renamed `$css` to `$value` for PHP 8 named parameter support.
 	 *
-	 * @param string $css The input value.
+	 * @param string $value CSS to update.
 	 * @return int|false The post ID or false if the value could not be saved.
 	 */
-	public function update( $css ) {
+	public function update( $value ) {
+		// Restores the more descriptive, specific name for use within this method.
+		$css = $value;
+
 		if ( empty( $css ) ) {
 			$css = '';
 		}
diff --git a/wp-includes/customize/class-wp-customize-header-image-setting.php b/wp-includes/customize/class-wp-customize-header-image-setting.php
index a45193f76e..7bbc923a40 100644
--- a/wp-includes/customize/class-wp-customize-header-image-setting.php
+++ b/wp-includes/customize/class-wp-customize-header-image-setting.php
@@ -24,7 +24,7 @@ final class WP_Customize_Header_Image_Setting extends WP_Customize_Setting {
 	 *
 	 * @global Custom_Image_Header $custom_image_header
 	 *
-	 * @param mixed $value
+	 * @param mixed $value The value to update.
 	 */
 	public function update( $value ) {
 		global $custom_image_header;
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 5af58b02b0..860fde97ec 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51783';
+$wp_version = '5.9-alpha-51784';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.