When building single-file web tools, personal dashboards, or local-first applications that don't need a backend server, the question of where to persist state comes up immediately. Options include: localStorage, sessionStorage, IndexedDB, cookies, Cache API, File System Access API, and even embedding state in the URL hash. Each has different size limits, APIs, and tradeoffs. The wrong choice leads to either over-engineering (setting up IndexedDB for 10KB of data) or hitting walls later (localStorage 5MB limit when storing images). This applies to: personal note-taking apps, bookmark managers, habit trackers, configuration tools, code snippet managers, and any tool where data lives on one device in one browser.