I use HTML Purifier to clean input in my CMS. Many of my clients like to have PayPal donation buttons in their site, but HTML Purifier strips the forms out.
I noticed that HTML Purifier has a HTMLPurifier_HTMLModule_Forms class, but I don't know if that does what I want. If it does, how do I enable it?
$config->set('HTML.Trusted', true);
$config->set('Attr.EnableID', true);
should turn on Forms (I think they’re enabled by default in the trusted set.) Of course, this also allows JavaScript too, so it might not be what you’re looking for...