The <content> element’s fallback content is displayed when its selector does not match anything. It would be useful if there was an element whose fallback content is displayed when the selector matches something, ie the fallback content is displayed in lieu of the selected content.

For example, you could make the labels of a tab strip this way:

<content select="h2:nth-of-type(-n+3)"></content>
<content select="h2:nth-of-type(-n+2)"></content>
<content select="h2:nth-of-type(-n+1)"></content>
<content magic="awesome" select="h2">
  …
</content>

The idea here is that the magic="awesome" attribute changes the behavior of the content element and the "…" indicator of more tabs would only be displayed if the selector *did* match more h2 elements.