On a drop-in host's dev server a missing posters/<id>.webp answered
200 text/html (the app's own page), and export handed that page to the
print dialog as the image. The .html source path had the same hole: the
rasteriser lifts the first <svg> on whatever page it is given.
library/packet/poster-fetch.js now fetches both. A raster counts only if
it is an image: by content-type, or by its first bytes when a host labels
a WebP application/octet-stream. A source counts only if it is a poster
document (its class="stage" element); a 200 that is not one reports 404.
A thrown fetch on the raster stays a miss, so the source fallback still
runs; a thrown fetch on the source is still reported as itself.
Measured against a real local HTTP server with a single-page-app fallback:
real WebP accepted (image/webp and octet-stream), missing WebP refused
(200 fallback page and 404), real source accepted, missing source 404
(200 fallback page and 404), unreachable host a miss for the raster and
"fetch failed" for the source.
usePosterPrints uses it. PosterWall follows in its own commit.
4fc4b8e9c8