diff -r 48c4eec2b7e6 -r 8c2e4d02f4ef wp/wp-includes/block-supports/background.php
--- a/wp/wp-includes/block-supports/background.php	Fri Sep 05 18:40:08 2025 +0200
+++ b/wp/wp-includes/block-supports/background.php	Fri Sep 05 18:52:52 2025 +0200
@@ -42,6 +42,7 @@
  * @since 6.4.0
  * @since 6.5.0 Added support for `backgroundPosition` and `backgroundRepeat` output.
  * @since 6.6.0 Removed requirement for `backgroundImage.source`. A file/url is the default.
+ * @since 6.7.0 Added support for `backgroundAttachment` output.
  *
  * @access private
  *
@@ -62,17 +63,19 @@
 		return $block_content;
 	}
 
-	$background_styles                    = array();
-	$background_styles['backgroundImage'] = isset( $block_attributes['style']['background']['backgroundImage'] ) ? $block_attributes['style']['background']['backgroundImage'] : array();
+	$background_styles                         = array();
+	$background_styles['backgroundImage']      = $block_attributes['style']['background']['backgroundImage'] ?? null;
+	$background_styles['backgroundSize']       = $block_attributes['style']['background']['backgroundSize'] ?? null;
+	$background_styles['backgroundPosition']   = $block_attributes['style']['background']['backgroundPosition'] ?? null;
+	$background_styles['backgroundRepeat']     = $block_attributes['style']['background']['backgroundRepeat'] ?? null;
+	$background_styles['backgroundAttachment'] = $block_attributes['style']['background']['backgroundAttachment'] ?? null;
 
 	if ( ! empty( $background_styles['backgroundImage'] ) ) {
-		$background_styles['backgroundSize']     = isset( $block_attributes['style']['background']['backgroundSize'] ) ? $block_attributes['style']['background']['backgroundSize'] : 'cover';
-		$background_styles['backgroundPosition'] = isset( $block_attributes['style']['background']['backgroundPosition'] ) ? $block_attributes['style']['background']['backgroundPosition'] : null;
-		$background_styles['backgroundRepeat']   = isset( $block_attributes['style']['background']['backgroundRepeat'] ) ? $block_attributes['style']['background']['backgroundRepeat'] : null;
+		$background_styles['backgroundSize'] = $background_styles['backgroundSize'] ?? 'cover';
 
 		// If the background size is set to `contain` and no position is set, set the position to `center`.
 		if ( 'contain' === $background_styles['backgroundSize'] && ! $background_styles['backgroundPosition'] ) {
-			$background_styles['backgroundPosition'] = 'center';
+			$background_styles['backgroundPosition'] = '50% 50%';
 		}
 	}