OPFS Origin Private File System
A private file system each site gets to itself, invisible to the user and with no dialogs. The thing that finally made real databases in the browser possible.
The web's earlier storage options were all poor for anyone wanting to implement something lower-level: key-value stores are asynchronous and have no notion of a position within a file, and the real file system requires the user to point at a folder. OPFS sits in between. It is reached without permission, is fully partitioned per origin, and contains ordinary files and directories that can be read and written at arbitrary offsets. The decisive addition is synchronous access handles, usable only inside a worker — and that is precisely the precondition for running compiled C code which expects blocking file operations. The most-discussed result is SQLite compiled to WebAssembly with actual persistence and reasonable performance. The storage counts as site data and therefore disappears when the user clears it.