mail.forwardemail.net

Open-Source & Privacy-focused Webmail: A privacy-first, offline-capable Progressive Web App for Forward Email. Ships as static assets and runs entirely in the browser with local caching, full-text search, and multi-account support. Maintained by forwardemail on GitHub, where it has earned 31 stars from the community.

It's actively developed around app, caching, javascript, and is a solid reference for anyone building with these tools.