Manual:$wgTidyConf

Details
May be set to the path of the custom configuration file for tidy if $wgUseTidy is set.

Before MediaWiki Version 1.10, this should be set to the actual location of the tidy configuration file, or a working configuration should be placed at $IP/extensions/tidy/tidy.conf - see Manual talk:$wgUseTidy for an example. Since Version 1.10 a working default configuration is provided at $IP/includes/tidy.conf.