Prioritize native lazy-loading, load lazysizes like polyfill

js/lazy.js

-(function () {
+(function (Drupal, drupalSettings) {
 
   'use strict';
 
-  function extend(obj, src) {
-    Object.keys(src).forEach(function (key) { obj[key] = src[key]; });
-    return obj;
-  }
+  Drupal.behaviors.lazy = {
+    attach: function attach(context, settings) {
+      var lazysizes = settings.lazy.lazysizes || {};
 
-  var lazysizes = drupalSettings.lazy.lazysizes || {};
+      if ('loading' in HTMLImageElement.prototype) {
+        var images = document.querySelectorAll('img[loading="lazy"]');
+        images.forEach(img => {
+          img.src = img.getAttribute(lazysizes.srcAttr);
+          this.removeClass(img, lazysizes.lazyClass);
+          this.addClass(img, lazysizes.loadedClass);
+        });
+      }
+      else {
+        window.lazySizesConfig = window.lazySizesConfig || {};
+        window.lazySizesConfig = this.extend(window.lazySizesConfig, lazysizes);
 
-  window.lazySizesConfig = window.lazySizesConfig || {};
-  window.lazySizesConfig = extend(window.lazySizesConfig, lazysizes);
+        var script = document.createElement('script'),
+          scripts = document.getElementsByTagName('script')[0];
+        script.src = Drupal.url('libraries/lazysizes/lazysizes.min.js');
+        scripts.parentNode.insertBefore(script, scripts);
+      }
+    },
+    extend: function extend(obj, src) {
+      Object.keys(src).forEach(function (key) { obj[key] = src[key]; });
+      return obj;
+    },
+    hasClass: function hasClass(el, className) {
+      return el.classList ? el.classList.contains(className) : new RegExp('\\b'+ className+'\\b').test(el.className);
+    },
+    addClass: function addClass(el, className) {
+      if (el.classList) el.classList.add(className);
+      else if (!hasClass(el, className)) el.className += ' ' + className;
+    },
+    removeClass: function removeClass(el, className) {
+      if (el.classList) el.classList.remove(className);
+      else el.className = el.className.replace(new RegExp('\\b'+ className+'\\b', 'g'), '');
+    }
+  }
 
-})();
+})(Drupal, drupalSettings);

lazy.libraries.yml

@@ -3,24 +3,8 @@ lazy:
   version: VERSION
   js:
     js/lazy.js: { }
-  dependencies:
-    - core/drupalSettings
-
-lazysizes:
-  header: true
-  remote: https://github.com/aFarkas/lazysizes
-  version: 5.1.1
-  license:
-    name: MIT
-    url: https://github.com/aFarkas/lazysizes/blob/master/LICENSE
-  js:
-    /libraries/lazysizes/plugins/respimg/ls.respimg.min.js: { minified: true }
-    /libraries/lazysizes/plugins/parent-fit/ls.parent-fit.min.js: { minified: true }
-    /libraries/lazysizes/plugins/object-fit/ls.object-fit.min.js: { minified: true }
-    /libraries/lazysizes/plugins/blur-up/ls.blur-up.min.js: { minified: true }
-    /libraries/lazysizes/lazysizes.min.js: { minified: true }
   css:
     theme:
       css/lazysizes.css: { }
   dependencies:
-    - lazy/lazy
+    - core/drupal