diff --git a/weasyprint/css/validation.py b/weasyprint/css/validation.py
index 98e40237..1d7ba76a 100644
--- a/weasyprint/css/validation.py
+++ b/weasyprint/css/validation.py
@@ -1651,7 +1651,7 @@ def preprocess_declarations(base_url, declarations):
 
         if name in PREFIXED and not name.startswith(PREFIX):
             validation_error(
-                'warn',
+                'warning',
                 'the property is experimental or non-standard, use '
                 + PREFIX + name)
             continue
@@ -1665,7 +1665,7 @@ def preprocess_declarations(base_url, declarations):
             unprefixed_name = name[len(PREFIX):]
             if unprefixed_name in UNPREFIXED:
                 validation_error(
-                    'warn',
+                    'warning',
                     'the property was unprefixed, use ' + unprefixed_name)
                 continue
             if unprefixed_name in PREFIXED:
@@ -1678,7 +1678,7 @@ def preprocess_declarations(base_url, declarations):
             result = list(expander_(base_url, name, tokens))
         except InvalidValues as exc:
             validation_error(
-                'warn',
+                'warning',
                 exc.args[0] if exc.args and exc.args[0] else 'invalid value')
             continue