A missing stylesheet can break the PDF export

You can reproduce this easily by creating a page with this content:

{{velocity output="false"}}
$xwiki.ssfx.use('missing.css')
{{/velocity}}

Some content.

If you try to export this page to PDF you get a timeout. This is caused by a paged.js bug, see https://github.com/pagedjs/pagedjs/issues/267 .