What Happens After You Type a URL Into the Browser, in Full Detail

Explain, in as much detail as you can, what happens from the moment a user types a URL such as `https://www.example.com/products?id=42` into a browser's address bar and presses Enter until the page is displayed and ready for interaction. The interviewer asked for as much detail as possible, so expect to be stopped and asked to go deeper at any step. ```hint Follow one request Walk the request outward from the browser (what it checks locally, how it finds the server, how it connects securely), then back in (what it does with the bytes it receives). ``` ```hint After the HTML arrives Ask what the browser must build before it can draw anything, and which resources can hold that up. ``` ### Clarifying Questions - Is this a first visit with empty caches, or a repeat visit? - Should I cover the server side (CDN, load balancers, application servers) as well as the network and the browser? - Should I assume HTTP/1.1, HTTP/2 or HTTP/3? ### What a Strong Answer Covers - Address bar handling: URL or search, parsing, HSTS, and local caches checked before any network traffic - Name resolution through browser, operating system and recursive resolver caches, with TTLs - Connection setup: TCP and TLS handshakes or QUIC, certificate validation, connection reuse - The HTTP exchange: request headers and cookies, CDN and load balancer, server processing, redirects and caching headers - Rendering: DOM and CSSOM construction, blocking scripts, layout, paint and compositing, and subresource loading - Where the latency goes, and what a repeat visit skips ### Follow-up Questions - Where would you look first to make this page load faster? - What changes with HTTP/2, and with HTTP/3? - How does the browser decide it may reuse a cached response, and when must it revalidate? - The host name resolves to several IP addresses and one of them is down. What happens?