In your bibliography, you cite a source. Link to it. When a reader double-clicks on the headline, the document expands, in place. Copy the citation into another outline, and you've got another link. Linking and outlining over the Internet. This is the start of something big." Me:
That said, I'm considering the additional of an embed tag (except that it's not a very good name) for version 1.1 of the DTD. The syntax, attributes and children would be the same as the xref element, except that there would be no expectation that the thingy on the other end is an otlml widget. How it is rendered would be left up to whatever is rendering the parent document; this would be a good place for individual document types to define entity declarations/attributes.
see also : <xsl:if test = "@style"> and XLink, Who Cares?