HTML Purifier 4.0 is a major feature release focused on configuration
It deprecates the $config->set('Ns', 'Directive', $value) syntax for
$config->set('Ns.Directive', $value); both syntaxes work but the
former will throw errors.  There are also some new features:  robust
support for name/id, configuration inheritance, remove nbsp in
the RemoveEmpty autoformatter, userland configuration directives
and configuration serialization.
