code: function checkIfStyleCanBeChildOf( def, currentParent, elementName, isUnknownElement ) {
  		return currentParent &&
  			( ( currentParent.getDtd() || CKEDITOR.dtd.span )[ elementName ] || isUnknownElement ) &&
  			( !def.parentRule || def.parentRule( currentParent ) );
  	}
docs: Check if the style element can be a child of the current node parent or if the element is not defined in the DTD.