diff --git a/addons/web/static/src/legacy/legacy_fields.scss b/addons/web/static/src/legacy/legacy_fields.scss
index 33d9188634f..8224d205d8f 100644
--- a/addons/web/static/src/legacy/legacy_fields.scss
+++ b/addons/web/static/src/legacy/legacy_fields.scss
@@ -1,3 +1,3 @@
-.o_legacy_field_widget > * {
+.o_debug .o_legacy_field_widget > * {
     background-color: rgba(255, 0, 0, 0.1);
 }
diff --git a/addons/web/static/src/legacy/scss/views.scss b/addons/web/static/src/legacy/scss/views.scss
index 5cc5407cc75..0f3bfa1b1a8 100644
--- a/addons/web/static/src/legacy/scss/views.scss
+++ b/addons/web/static/src/legacy/scss/views.scss
@@ -3,7 +3,7 @@
  * all renderers. For field default rules, see the fields.scss file.
  */
 
-.o_legacy_kanban_view {
+.o_debug .o_legacy_kanban_view {
     border-top: 2px solid red;
 }
 
diff --git a/addons/web/static/src/start.js b/addons/web/static/src/start.js
index f0b3361cd30..89e04c8bbec 100644
--- a/addons/web/static/src/start.js
+++ b/addons/web/static/src/start.js
@@ -56,6 +56,9 @@ export async function startWebClient(Webclient) {
     if (env.services.user.userId === 1) {
         classList.add("o_is_superuser");
     }
+    if (env.debug) {
+        classList.add("o_debug");
+    }
     // delete odoo.debug; // FIXME: some legacy code rely on this
     odoo.__WOWL_DEBUG__ = { root };
     odoo.isReady = true;