diff --git a/addons/mass_mailing/static/src/js/mass_mailing_design_constants.js b/addons/mass_mailing/static/src/js/mass_mailing_design_constants.js
index b692acd0678..cf6fbe0dbf7 100644
--- a/addons/mass_mailing/static/src/js/mass_mailing_design_constants.js
+++ b/addons/mass_mailing/static/src/js/mass_mailing_design_constants.js
@@ -28,7 +28,7 @@ export const PRIORITY_STYLES = {
     'a:not(.btn)': [],
     'a.btn.btn-primary': [],
     'a.btn.btn-secondary': [],
-    'hr': ['border-top-color'],
+    'hr': ['border-top-width','border-top-style','border-top-color'],
 };
 export const RE_CSS_TEXT_MATCH = /([^{]+)([^}]+)/;
 export const RE_SELECTOR_ENDS_WITH_GT_STAR = />\s*\*\s*$/;