If you have a reference to the Child layer and need to get an object in the Parent layer, use the parentLayer property of the Child layer object to retrieve the Parent layer object, then use the Parent layer's document property to find objects contained within it. This gets the document's immediately containing layer object. In general, it is also possible to reference objects in any layer in the document by starting with the top level array of layers and navigating down into nested layers level by level until the desired layer and object are located. Currently, there are a few restrictions on the kind of file which can be specified. It's not yet possible to use an image or a page that contains frames as the SRC of a layer.
Layers are transparent by default.
The use of transparent GIFs is also supported. This kind of translucency applies to the plugin rather than to HTML content. Danny Goodman has written an excellent article about supporting multiple browser versions in View Source, Netscape's online magazine. It will probably still work in the preview releases. It may be removed from the final release.