This concept then got extended and in February 1999, I wrote the following to the xml-dev mailing list:
For a while now, I've been thinking what an OS (or more likely shell) would look like if it took Unix's "everything as a file" to "everything as an XML element".
A system would be a single XML "uberdocument" (physically, separate entities, including unparsed for any non-XML files on the system but logically, the one XML document). Applications (which would themselves be nodes in the element tree) would operate on other nodes in the element tree.
There would be an application, for example, that got mail via POP or IMAP, represented it in XML and then attached it a particular point in the uberdocument. XSL could be used to sort the mail. XSL would also be used to view the mail.
It's XML for the sake of it, but I think it would be fun to try out.