koa-responsive-image-router is a powerful library for adding responsive images to a server-side rendered app, without any need for JS on the front-end. Its features include:
- serving images in multiple modern formats;
- generating lowres thumbnails that load before the full image is downloaded;
- integration with smartcrop.js (GitHub - jwagner/smartcrop.js: Content aware image cropping);
- smart cache.
Using responsive images properly is hard (see ✩ Responsywne obrazki w HTML) and this library makes it easier.