Manual:$wgAlwaysUseTidy

Details
Obviously it relates to $wgUseTidy and the ability to run 'HTML tidy' tool over the parser output.

In Parser.php line 357:

...but what is $this->mOptions ? This implies that it wont always run HTML tidy over parser output, but will allow this to be switched off via some parser options mechanism.... unless you set $wgAlwaysUseTidy=true;