diff --git a/addons/web/views/report_templates.xml b/addons/web/views/report_templates.xml
index f60077a334f..a7df246da9b 100644
--- a/addons/web/views/report_templates.xml
+++ b/addons/web/views/report_templates.xml
@@ -15,6 +15,16 @@
                 <meta name="viewport" content="initial-scale=1"/>
                 <title><t t-esc="title or 'Odoo Report'"/></title>
                 <t t-call-assets="web.report_assets_common"/>
+                <!-- Temporary code: only used to maintain CSS for legacy HTML reports (full width...) -->
+                <!-- Should be removed once the reports are fully converted. -->
+                <script type="text/javascript">
+                    document.addEventListener('DOMContentLoaded', () => {
+                        if (window.self !== window.top) {
+                            document.body.classList.add("o_in_iframe", "container-fluid");
+                            document.body.classList.remove("container");
+                        }
+                    });
+                </script>
             </head>
             <body t-att-class="'container' if not full_width else 'container-fluid'">
                 <div id="wrapwrap">