#3167034 "Leverage the 'loading' html attribute to enable lazy-load by default for images"

Related #3167034

core/includes/theme.inc

@@ -851,6 +851,13 @@ function template_preprocess_image(&$variables) {
       $variables['attributes'][$key] = $variables[$key];
     }
   }
+
+  // Without dimensions specified, layout shifts can occur,
+  // which are more noticeable on pages that take some time to load.
+  // As a result, only mark images as lazy load that have dimensions.
+  if (isset($variables['width'], $variables['height']) && !isset($variables['attributes']['loading'])) {
+    $variables['attributes']['loading'] = 'lazy';
+  }
 }
 
 /**