A better approach would be to allow this as a special type of URL, so
that normal HREFs could be used. This does not require any changes to
the spec's, and will parse without problem in current browsers. Note
that it will not perform as expected in current browsers, beyond
telling them that they don't understand the "browser:///back"
protocol.
An even better solution to all this is the use of LINK to allow
documents to add buttons to a doc-specific button-bar. That is, after
all, why they are there.
Christian "This UI mess is all Mosaic's fault!"