diff --git a/odoo/addons/base/models/assetsbundle.py b/odoo/addons/base/models/assetsbundle.py
index 345ab279e32..eaaaa2f54a0 100644
--- a/odoo/addons/base/models/assetsbundle.py
+++ b/odoo/addons/base/models/assetsbundle.py
@@ -452,9 +452,9 @@ class AssetsBundle(object):
         # Post process the produced css to add required vendor prefixes here
         compiled = re.sub(r'(appearance: (\w+);)', r'-webkit-appearance: \2; -moz-appearance: \2; \1', compiled);
 
-        compiled = re.sub(r'(display: ((?:inline-)?)flex((?: ?!important)?);)', r'display: -webkit-\2flex\3; \1', compiled)  # For PhantomJS tests
+        compiled = re.sub(r'(display: ((?:inline-)?)flex((?: ?!important)?);)', r'display: -webkit-\2box\3; display: -webkit-\2flex\3; \1', compiled)  # For PhantomJS tests and wkhtmltopdf
         compiled = re.sub(r'(flex-flow: (\w+ \w+);)', r'-webkit-flex-flow: \2; \1', compiled) # For PhantomJS tests
-        compiled = re.sub(r'(flex: (\d+ \d+ (?:\d+|auto));)', r'-webkit-flex: \2; \1', compiled)  # For PhantomJS tests
+        compiled = re.sub(r'(flex: ((\d)+ \d+ (?:\d+|auto));)', r'-webkit-box-flex: \3; -webkit-flex: \2; \1', compiled)  # For PhantomJS tests and wkhtmltopdf
 
         return compiled