Web & Networking › How Browsers Work
Web Browser
The program that fetches, renders and runs web pages.
Also known as: web browser, Chrome, Firefox, Safari
A web browser is the program that fetches web pages, renders them and runs their JavaScript. Chrome, Safari, Firefox and Edge are the common ones.
Inside, a browser does several jobs:
- Networking: requests pages and resources over HTTP and HTTPS (what happens when you type a URL).
- Parsing and rendering: turns HTML and CSS into the DOM and the pixels on screen (critical rendering path).
- Running code: a JavaScript engine executes scripts.
- Enforcing security: the same-origin policy, sandboxing, HTTPS checks.
- Storing data: caches, cookies and storage (browser storage).
- Developer tools: built-in inspection and debugging (DevTools).
Engines
Browsers share a few rendering engines:
| Engine | Used by |
|---|---|
| Blink | Chrome, Edge, Opera, Brave and others |
| WebKit | Safari (and every browser on iOS) |
| Gecko | Firefox |
So there are really three engines to test against (browser compatibility).
Why developers care
- Your frontend runs on machines you don’t control: different browsers, versions, screen sizes and settings.
- The browser is a hostile environment for secrets. Anything sent to it can be read by the user.
- Behavior differs around features, caching and security. Test on the ones your users have.
- DevTools are your main debugging tool.
The browser’s security rules (origins, CORS, cookies, CSP) explain many “why won’t this request work?” problems (CORS).