What Happens When You Type a URL?

Problem Walk through, step by step, everything that happens from typing a URL into the browser to the page rendering on screen.

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• URL parsing and preprocessing: scheme/host/path split, HSTS check, and the browser deciding whether this is a search query or a navigation.
• DNS resolution: browser cache, OS cache, hosts file, then the recursive resolver walking root, TLD, and authoritative nameservers; TTLs and DNS caching.
• TCP connection: the three-way handshake to the resolved IP, plus connection reuse and the role of QUIC/HTTP3 where UDP replaces it.
• TLS handshake for HTTPS: certificate chain validation, key exchange, cipher negotiation, ALPN, and session resumption.
• The HTTP request/response: request line and headers, cookies, the path through CDN, load balancer, and gateway to the origin server, and the status code that comes back.
• HTML parsing and the DOM: incremental parsing, how blocking scripts stall the parser, and async/defer.
• CSSOM and the render tree: stylesheets block rendering, the render tree combines DOM + CSSOM.
• Layout, paint, and composite: computing geometry, rasterising, and compositing layers on the GPU; where reflow and repaint fit.
• Extras worth naming: caching headers and conditional requests, critical rendering path, and Time to First Byte.