diff --git a/addons/web/static/src/legacy/js/public/lazyloader.js b/addons/web/static/src/legacy/js/public/lazyloader.js
index e4c98b2ec38..5182d92bd30 100644
--- a/addons/web/static/src/legacy/js/public/lazyloader.js
+++ b/addons/web/static/src/legacy/js/public/lazyloader.js
@@ -103,6 +103,7 @@ function _loadScripts(scripts, index) {
     }
     var script = scripts[index];
     script.addEventListener('load', _loadScripts.bind(this, scripts, index + 1));
+    script.setAttribute('defer', 'defer');
     script.src = script.dataset.src;
     script.removeAttribute('data-src');
 }
diff --git a/odoo/addons/base/models/assetsbundle.py b/odoo/addons/base/models/assetsbundle.py
index d7cd7de9b85..3759ad9982e 100644
--- a/odoo/addons/base/models/assetsbundle.py
+++ b/odoo/addons/base/models/assetsbundle.py
@@ -191,7 +191,9 @@ class AssetsBundle(object):
             src = self.get_debug_asset_url(name=js_attachment.name, extension='') if is_debug_assets else js_attachment[0].url
             attr = dict([
                 ["async", "async" if async_load else None],
-                ["defer", "defer" if defer_load or lazy_load else None],
+                # lazy_load will add defer in JS otherwise this is not W3C valid
+                # (defer is probably not needed there anyways)
+                ["defer", "defer" if defer_load else None],
                 ["type", "text/javascript"],
                 ["data-src" if lazy_load else "src", src],
                 ['data-asset-bundle', self.name],
diff --git a/odoo/addons/base/models/ir_qweb.py b/odoo/addons/base/models/ir_qweb.py
index d6f7755a659..8b7a3e4e155 100644
--- a/odoo/addons/base/models/ir_qweb.py
+++ b/odoo/addons/base/models/ir_qweb.py
@@ -2512,7 +2512,9 @@ class IrQWeb(models.AbstractModel):
                         "type": mimetype,
                     }
                     attributes["data-src" if lazy_load else "src"] = path
-                    if defer_load or lazy_load:
+                    if defer_load:
+                        # lazy_load will add defer in JS otherwise this is not
+                        # W3C valid (defer is probably not needed there anyways)
                         attributes["defer"] = "defer"
                 elif is_css:
                     tag = 'link'