What happens when you hit a URL — Critical Rendering Path explained

Browser does DNS → TCP → TLS → HTTP, then parses HTML into the DOM. CSS builds the CSSOM (render-blocking). DOM + CSSOM → render tree → layout → paint → composite. Synchronous JS blocks the parser; defer/async unblock it.

7 min read·~15 min to think through

When the user hits enter on a URL, the browser executes a long chain of work. The interview-friendly summary: Network → Parse → Render. The senior detail is what blocks what — that's where every perf win comes from.

Phase 1: Network.

DNS lookup. Resolve the hostname. Cached in OS, browser, router. ~20–120ms cold.
TCP handshake. 3-way SYN/SYN-ACK/ACK. ~100ms RTT.
TLS handshake. Certificate exchange + key agreement. 1–2 RTT (TLS 1.3 reduced this).
HTTP request. Send GET, wait for first byte (TTFB). Server processes, possibly hits a CDN or origin.
Response. Bytes stream back. With HTTP/2 multiplexing, more resources can come over the same connection.

Phase 2: Parse.

HTML parser turns bytes into the DOM tree incrementally as bytes arrive.
When the parser hits a <link rel="stylesheet">, it requests the CSS but continues parsing HTML in parallel.
CSS is parsed into the CSSOM (CSS Object Model). CSS is render-blocking — the browser won't paint anything until CSSOM is built (otherwise unstyled content would flash).
When the parser hits a <script>:

Default: parser pauses, fetches and executes the script before continuing. Worst case for performance.
async: fetched in parallel; executes whenever it arrives, possibly mid-parse. Order not guaranteed.
defer: fetched in parallel; executes after parsing is complete, in order. Best default for most scripts.
type="module": defer-like by default.

Phase 3: Render.

Render tree combines DOM + CSSOM, dropping invisible nodes (display: none, <head>, etc.).
Layout (reflow). Compute geometry of every render-tree node.
Paint. Fill in pixels into layer bitmaps.
Composite. GPU assembles layers and shows the frame.

Render-blocking vs parser-blocking.

Render-blocking = blocks the first paint. CSS by default; sync scripts; web fonts (FOIT).
Parser-blocking = blocks DOM construction. Sync scripts in particular — async/defer fix this.

The optimizations the CRP teaches you.

Inline critical CSS above the fold so the first paint doesn't wait for an external stylesheet.
<script defer> for app code; the parser keeps going.
<link rel="preload"> for late-discovered resources (font referenced from CSS, JS chunk).
<link rel="preconnect"> to third-party origins to skip DNS+TCP+TLS later.
Server push / 103 Early Hints to start asset fetches before the HTML response body arrives.
HTTP/2 (or HTTP/3) for multiplexing — many requests over one connection.
Brotli/Gzip to shrink what's on the wire.
HTML streaming + early flush so the browser starts parsing before the server is done generating.

Where the milliseconds go (rough budget for a fast site on a fast network):

DNS+TCP+TLS: 100–300ms (warmable with preconnect).
TTFB: 100–500ms (cacheable with CDN edge).
HTML download: 50–200ms.
CSS download + parse: 100–300ms.
JS download + parse + execute: 100–1000ms+ (the usual culprit).
Layout + paint of first frame: 50–200ms.

Web Vitals map cleanly onto CRP.

TTFB measures the network phase up to the first response byte.
FCP (First Contentful Paint) — first non-empty paint, requires HTML parsed enough + CSSOM built + render done.
LCP (Largest Contentful Paint) — bigger render, often gated on the hero image (preload + dimensions help).
CLS — caused by content shifting after first paint (images without dimensions, late-loaded fonts swapping).
INP — long tasks blocking the main thread after first paint (heavy JS).

Mental model in one line: the browser races to build DOM + CSSOM, the first paint waits for both, and JS controls how much of that race it slows down.

Code

Follow-up questions

•Why is CSS render-blocking?
•What's the difference between async and defer?
•How does inlining critical CSS speed up FCP?
•What does TTFB include?

Common mistakes

•Putting scripts in <head> without async/defer — blocks DOM construction.
•Treating CSS as non-blocking — it is, by default.
•Loading too many third-party scripts synchronously — kills FCP.
•Forgetting that web fonts can FOIT (block text rendering) without `font-display: swap`.

Performance considerations

•First paint is gated on CSSOM — keep critical CSS small and inlined.
•Streaming HTML + early flush gives the browser something to parse sooner.
•Reduce JS on the critical path — every KB delays interactivity.

Edge cases

•<link rel='preload'> without `as` — counted but downloaded again.
•Service worker can intercept and serve from cache, short-circuiting most of phase 1.
•HTTP/2 server push exists but is largely deprecated in favor of 103 Early Hints.

Real-world examples

•Next.js streams HTML with React 18 to start CRP before the full page is generated.
•Cloudflare Early Hints sends 103 with link preloads while the origin is still working.

Senior engineer discussion

Senior signal: naming the phases, what's render-blocking vs parser-blocking, and connecting CRP knobs (defer, preload, critical CSS) to specific Web Vitals.