diff --git a/addons/mass_mailing/data/mail_data.xml b/addons/mass_mailing/data/mail_data.xml
index 6c9bd7b591d..9b7fd81ce70 100644
--- a/addons/mass_mailing/data/mail_data.xml
+++ b/addons/mass_mailing/data/mail_data.xml
@@ -1,6 +1,9 @@
 <?xml version="1.0" encoding="utf-8"?>
 <odoo><data noupdate="1">
-    <!-- Reference: https://litmus.com/community/learning/24-how-to-code-a-responsive-email-from-scratch -->
+    <!--
+    Reference: https://litmus.com/community/learning/24-how-to-code-a-responsive-email-from-scratch
+    https://www.campaignmonitor.com/css/link-element/link-in-head/
+    -->
     <template id="mass_mailing_mail_layout">
         &lt;!DOCTYPE html&gt;
         <html xmlns="http://www.w3.org/1999/xhtml">
diff --git a/addons/mass_mailing/static/tests/field_html_test.js b/addons/mass_mailing/static/tests/field_html_test.js
new file mode 100644
index 00000000000..49c38244089
--- /dev/null
+++ b/addons/mass_mailing/static/tests/field_html_test.js
@@ -0,0 +1,4 @@
+/** @odoo-module alias=mass_mailing.FieldHtml.test */
+import MassMailingFieldHtml from "mass_mailing.FieldHtml";
+
+MassMailingFieldHtml.include({ jsLibs: [] });
diff --git a/addons/mass_mailing/tests/test_mailing_internals.py b/addons/mass_mailing/tests/test_mailing_internals.py
index 658d1eaf63c..4639815bbb7 100644
--- a/addons/mass_mailing/tests/test_mailing_internals.py
+++ b/addons/mass_mailing/tests/test_mailing_internals.py
@@ -20,7 +20,13 @@ class TestMassMailValues(MassMailCommon):
 
     @users('user_marketing')
     def test_mailing_body_responsive(self):
-        """ Testing mail mailing responsive mail body """
+        """ Testing mail mailing responsive mail body
+
+        Reference: https://litmus.com/community/learning/24-how-to-code-a-responsive-email-from-scratch
+        https://www.campaignmonitor.com/css/link-element/link-in-head/
+
+        This template is meant to put inline CSS into an email's head
+        """
         recipient = self.env['res.partner'].create({
             'name': 'Mass Mail Partner',
             'email': 'Customer <test.customer@example.com>',
@@ -48,6 +54,7 @@ class TestMassMailValues(MassMailCommon):
         self.assertIn('<!DOCTYPE html>', body_html)
         self.assertIn('<head>', body_html)
         self.assertIn('viewport', body_html)
+        # This is important: we need inline css, and not <link/>
         self.assertIn('@media', body_html)
         self.assertIn('I am Responsive body', body_html)
 
diff --git a/addons/mass_mailing/views/assets.xml b/addons/mass_mailing/views/assets.xml
index 96dc4fcc101..574f0dddc6f 100644
--- a/addons/mass_mailing/views/assets.xml
+++ b/addons/mass_mailing/views/assets.xml
@@ -17,7 +17,6 @@
     <template id="assets_mail_themes">
         <link rel="stylesheet" type="text/scss" href="/mass_mailing/static/src/scss/themes/theme_basic.scss"/>
         <link rel="stylesheet" type="text/scss" href="/mass_mailing/static/src/scss/themes/theme_default.scss"/>
-        <t t-call="mass_mailing.mass_mailing_mail_style"/>
     </template>
 
     <template id="assets_mail_themes_edition"> <!-- maybe to remove and convert into a field dumy with attr invisible if the template is not selected -->
@@ -29,6 +28,7 @@
 
     <template id="iframe_css_assets_edit" groups="base.group_user">
         <t t-call-assets="mass_mailing.assets_mail_themes" t-js="false"/>
+        <t t-call="mass_mailing.mass_mailing_mail_style"/>
         <t t-call-assets="mass_mailing.assets_mail_themes_edition" t-js="false"/>
     </template>
 
@@ -44,19 +44,19 @@
 
     <template id="qunit_suite" inherit_id="web.qunit_suite_tests">
         <xpath expr="." position="inside">
-            <script type="text/javascript">
-                odoo.define('mass_mailing.FieldHtml.test', function (require) {
-                    'use strict';
-                    var MassMailingFieldHtml = require('mass_mailing.FieldHtml');
-                    MassMailingFieldHtml.include({jsLibs: []});
-                });
-            </script>
+            <script type="text/javascript" src="/mass_mailing/static/tests/field_html_test.js"/>
             <script type="text/javascript" src="/mass_mailing/static/src/js/mass_mailing_snippets.js"/>
             <script type="text/javascript" src="/mass_mailing/static/tests/mass_mailing_html_tests.js"/>
         </xpath>
     </template>
 
     <template id="mass_mailing_mail_style">
+        <!--
+        Reference: https://litmus.com/community/learning/24-how-to-code-a-responsive-email-from-scratch
+        https://www.campaignmonitor.com/css/link-element/link-in-head/
+
+        This template is meant to put inline CSS into an email's head
+        -->
         <style>
             @media screen and (max-width: 768px) {
                 .o_mail_col_mv {