Priority 1 (highest)
-Investigate and fix the crash that occurs after we trigger exit from null_prompt_service.
Priority 2
Use ffmpeg to convert unsuitable video files (how do we check this?).
For any page with no links, add a "back" link (this raises the issue of localisation, of course).
If a page is too large, split it and add "next" and "previous" links (seems very difficult).
Set Mozilla's idea of the pixel aspect ratio to match video pixels. (For extra credit, scale preview mode to correct for the differing aspect ratio.) This seems to be impossible.
+Avoid running Xvfb if we're only going to display an error message.