When trying to screenshot, test, or automate interactions with a local HTML file via Playwright (including Playwright MCP), navigating to file:// URLs fails with a security error or the page loads blank. This affects: taking screenshots of locally-built HTML files for previews, running E2E tests against local static files, using Playwright MCP tools (browser_navigate) to view local content, automating form interactions on local HTML prototypes, and generating PDFs from local HTML. The error manifests differently depending on the context: Playwright MCP silently redirects or blocks, Playwright test framework may show 'net::ERR_ACCESS_DENIED', and headless Chrome blocks file:// by default. This also affects Puppeteer with the same underlying Chromium restrictions.

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.