1. HTML, CSS, and JS Loading

• When you open a website, the browser downloads the HTML, CSS, and JS files.

• HTML creates the structure, CSS styles it, and JavaScript makes it interactive.

2. Parsing and Rendering

• The browser first parses the HTML and builds a DOM (Document Object Model) — a tree structure representing the page.

• It also builds a CSSOM (CSS Object Model) from CSS.

• Then, it combines DOM + CSSOM into a Render Tree to paint pixels on the screen.

3. JavaScript Engine

• The browser has a JavaScript engine (e.g., Chrome has V8, Firefox has SpiderMonkey).

• JS code is parsed, compiled (nowadays to machine code for speed), and executed by the engine.

4. Single-Threaded Event Loop

• JavaScript runs in a single thread inside the browser.

• It uses an event loop to handle tasks:

  • Call Stack: Executes functions one by one.

  • Web APIs: Browser provides APIs like setTimeout, fetch, DOM manipulation.

  • Callback Queue: Stores tasks (e.g., click events, API responses) and feeds them back to the call stack when it's empty.

5. Non-blocking Behavior

• While JS is single-threaded, the browser handles tasks like AJAX, timers, and user input asynchronously via Web APIs and the event loop. So your page doesn’t freeze.

6. Repainting and Reflow

• If JS changes the DOM or styles, the browser may do a reflow (layout calculation) and repaint the page.

7. Modern Enhancements

• Browsers now also use Web Workers for running heavy JS in the background.

• JS modules and Promises/async-await help in managing code cleanly and asynchronously.