{# A backdrop (docs/backgrounds.md): the scene the page's
`#backdrop-data` names (written just before this include) drawn by the
engine, Rust compiled to WebAssembly. The viewer's choice (Settings › Appearance) is a cookie;
unset, a touch-only device or Save-Data gets a still sky. Any failure
leaves no backdrop and nothing else changed. A sky with `live` polls
`/sky/arrivals` for what happened since and adds it. #}