Choose a scenario or paste a URL, then step through how the browser turns navigation into DNS lookup, TLS, HTTP, cache checks, parsing, and first paint.
Web Loading Lab
Choose a scenario or paste a URL, then step through how the browser turns navigation into DNS lookup, TLS, HTTP, cache checks, parsing, and first paint.
About browser loading simulation
A controllable model, not a live fetch
The lab uses hand-authored scenarios to make the browser loading pipeline visible. It does not resolve DNS, open sockets, request remote pages, or measure live timing.
See how one setting changes the whole trip
Cache hit skips the network path, redirects add another request, 304 reuses cached bytes, and non-HTML responses change what the browser can render. Those changes are shown directly on the step timeline.
Local-only and safe for internal URLs
The URL and headers stay in the browser state. They are used only to generate a teaching journey and are not sent to a server.
FAQ
Common questions and answers about this topic.
How do I use the Web Loading Lab?
Choose a preset scenario or paste a URL, adjust cache, redirect, TLS, status, and content-type settings, then press Next or Play to step through the browser loading journey.
Does this tool test a real website?
No. It is a teaching simulator. It does not send network requests, measure speed, or inspect a live server. Use browser DevTools, Lighthouse, WebPageTest, or server logs for real diagnostics.
Why does cache hit skip DNS, TLS, and HTTP request steps?
In this simplified model, a fresh cached response can be used locally, so the browser does not need a new network trip. Revalidation is different: it still contacts the server and may receive 304.
What changes when a page redirects?
The browser first receives a 3xx response with a Location header, then starts another request to the target URL. That usually adds at least one extra round trip before parsing can begin.
What is the TLS step doing?
TLS lets the browser verify the server certificate and agree on encrypted communication keys before sending HTTP data over HTTPS. A resumed TLS session can reduce the handshake work.
Is first paint the same as a fully loaded page?
No. First paint means the browser has enough information to draw something. Images, fonts, scripts, hydration, and later network requests may still continue after that.