Origin Private File System (OPFS)
A high-speed, private file system that a website gets in the browser, isolated per origin and invisible to the user. Designed for performance — it gives apps a real file system to write large amounts of data quickly, e.g. to run a whole database in the browser.
Background: web apps needed better storage than IndexedDB's object store and localStorage for heavy cases — e.g. running SQLite or a media editor in the browser. OPFS gives a sandboxed file system (per origin, not connected to the user's regular files) with normal file operations, and — crucially — a synchronous access variant (createSyncAccessHandle) that runs in a worker and is much faster than the asynchronous alternatives. Use case: SQLite-in-the-browser (sql.js/wa-sqlite against OPFS), video/image editing, large datasets, local-first apps that need fast persistent storage. It differs from the File System Access API (which touches the user's real files with a dialog) — OPFS is private and requires no permission. The modern basis for serious client storage. Related to cache storage API and IndexedDB.