diff --git a/addons/mass_mailing/static/src/js/mass_mailing_widget.js b/addons/mass_mailing/static/src/js/mass_mailing_widget.js
index 9312ffc3053..0eaf9a4077e 100644
--- a/addons/mass_mailing/static/src/js/mass_mailing_widget.js
+++ b/addons/mass_mailing/static/src/js/mass_mailing_widget.js
@@ -115,6 +115,10 @@ var MassMailingFieldHtml = FieldHtml.extend({
         this.$content.find('.o_layout').addBack().data('name', 'Mailing');
         // We don't want to drop snippets directly within the wysiwyg.
         this.$content.removeClass('o_editable');
+        // Force compute CSS rules even without the style-inline node option.
+        if (this.mode === 'edit' && !this.cssRules) {
+            this.cssRules = convertInline.getCSSRules(this.wysiwyg.getEditable()[0].ownerDocument);
+        }
     },
     /**
      * Returns true if the editable area is empty.
diff --git a/addons/web_editor/static/src/js/backend/convert_inline.js b/addons/web_editor/static/src/js/backend/convert_inline.js
index 370dd208d55..0974748bd9f 100644
--- a/addons/web_editor/static/src/js/backend/convert_inline.js
+++ b/addons/web_editor/static/src/js/backend/convert_inline.js
@@ -955,7 +955,9 @@ FieldHtml.include({
 
     _createWysiwygIntance: function () {
         return this._super(...arguments).then(() => {
-            this.cssRules = getCSSRules(this.wysiwyg.getEditable()[0].ownerDocument);
+            if (this.nodeOptions['style-inline'] && this.mode === "edit") {
+                this.cssRules = getCSSRules(this.wysiwyg.getEditable()[0].ownerDocument);
+            }
         });
     },