This will allow per-work targeting of CSS to the export environment only.

Ideally there would be two classes:

• ws-export: indicating any export environment
• ws-export-{format}: for the exact format (PDF, epub, etc)

This would allow, for example, selective CSS like:

body.ws-export-epub .wst-sidenote {
    display: none; /* or display: block;, etc etc */
}