filter: url() question

A question about the url() syntax for CSS Filter Effects:  Should SVG nodes
in an external file referenced by url() be added to the DOM?  Ie., should
it be possible to find them by ID from JavaScript?

For example, the Firefox implementation of this feature seems to work for
SVG nodes contained in the same file (since they're already in the DOM at
load time, getElementById() works), but not for externally-referenced
files.  The filter is applied, but getElementById() returns NULL, even
for the filter node whose ID was used as the fragment identifier in the URL.

Is this the intended behaviour?

Stephen