This should probably be specified.

Options I can think of:

1. The fallback content, e.g. <portal src="data:text/foo,bar">This will be displayed</portal>.

2. A transparent box of the same size as the portal.

3. Generate no boxes, as-if display: none.

4. A user-agent-defined error signifier, like a broken image or network-error iframe.

At first (1) felt natural, but I think it'd probably be more harmful to developers than helpful. In particular fallback content is mostly useful for "your browser does not support portals". Cases of "your browser does support portals, but there was an error displaying the content" seem different. So I lean toward one of the other options.