How did it slip in there? I can’t really say. All I can say is, that it got included with an entry for Apples’s PropertyList DTD. But how did I know then, there is such an attribute with such a value?
I removed it now, actually I put it inside an XML comment together with some words explaining my confusion. In case this action now will break any processing in the future, I hope, I will find this blog article then.