http://www.xulplanet.com/tutorials/xultu/advrules.html http://www.xulplanet.com/reference/elemref/ref_member.html Used within a rule's conditions element to match elements that are containers or are contained within another element. Both the container and child attributes may use variables. A variable reference to a node that is expected to be a child of the element that is referred to by the container element. A variable reference to a node that is expected to be a container. If the variable is the same as the uri attribute on the content element, the resource must be a container element in order to match.