Fix output buffering hooks to locate sources by wrapping hooks after …

…*after* overall buffer

includes/class-amp-theme-support.php

@@ -263,6 +263,11 @@ public static function add_hooks() {
 		 */
 		add_action( 'template_redirect', array( __CLASS__, 'start_output_buffering' ), 0 );
 
+		// Add validation hooks *after* output buffering has started for the response.
+		if ( AMP_Validation_Utils::should_validate_response() ) {
+			add_action( 'template_redirect', array( 'AMP_Validation_Utils', 'add_validation_hooks' ), 1 );
+		}
+
 		// Commenting hooks.
 		add_filter( 'wp_list_comments_args', array( __CLASS__, 'set_comments_walker' ), PHP_INT_MAX );
 		add_filter( 'comment_form_defaults', array( __CLASS__, 'filter_comment_form_defaults' ) );
@@ -272,10 +277,6 @@ public static function add_hooks() {
 		remove_action( 'comment_form', 'wp_comment_form_unfiltered_html_nonce' );
 		add_filter( 'wp_kses_allowed_html', array( __CLASS__, 'whitelist_layout_in_wp_kses_allowed_html' ), 10 );
 
-		if ( AMP_Validation_Utils::should_validate_response() ) {
-			AMP_Validation_Utils::add_validation_hooks();
-		}
-
 		// @todo Add character conversion.
 	}