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.