diff --git a/lib/commonmarker/config.rb b/lib/commonmarker/config.rb
index 70f1e6ba..f419bb45 100644
--- a/lib/commonmarker/config.rb
+++ b/lib/commonmarker/config.rb
@@ -46,6 +46,7 @@ module Config
         spoiler: false,
         greentext: false,
       }.freeze,
+      format: [:html].freeze,
     }.freeze
 
     PLUGINS = {