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How a Browser Works A Beginner-Friendly Guide to Browser Internals

Published: (January 31, 2026 at 03:52 AM EST)
4 min read
Source: Dev.to

Source: Dev.to

What Really Happens When You Type www.twitter.com and Hit Enter?

You’ve probably typed www.twitter.com into your browser thousands of times.
Twitter opens almost instantly… but have you ever wondered what actually happens inside the browser during those few milliseconds?

A browser doesn’t just “open a website.” It goes on a fast, complex journey to fetch, understand, organize, and finally display the page on your screen. In this guide we’ll break that journey down step by step — without unnecessary jargon.

What Is a Browser?

A browser is an application that allows users to access information on the internet by acting as a bridge between the user and the server.

Popular browsers include:

  • Google Chrome
  • Safari
  • Firefox
  • Microsoft Edge
  • Brave
  • Opera

When you search or open a website in any of these browsers, a lot happens behind the scenes in milliseconds.

The Main Parts of a Browser (High-Level)

Think of a browser like a team of workers, each with a specific role.

1. User Interface (UI)

Everything you interact with:

  • Address bar
  • Tabs
  • Back/forward buttons
  • Bookmarks

Note: The UI does not display the website itself — it’s only for interaction.

2. Browser Engine

The coordinator that connects the UI with the Rendering Engine and tells other components what to do and when.

3. Rendering Engine

The artist that turns raw HTML and CSS into the visual webpage you see.

  • Chrome → Blink
  • Firefox → Gecko
  • Safari → WebKit

4. JavaScript Engine

The specialist that executes JavaScript code and handles logic, events, and dynamic behavior.

  • Chrome → V8
  • Firefox → SpiderMonkey

5. Networking

The delivery team responsible for fetching HTML, CSS, JavaScript, images, fonts — everything needed from the server.

6. Data Storage

The storeroom that holds:

  • Cache
  • Cookies
  • LocalStorage
  • SessionStorage

This helps make future visits faster.

7. Graphics / UI Backend

The painter that draws text, images, shapes, and layouts on your screen using low‑level graphics APIs.

High-Level Browser Flow

UI
 ↓
Browser Engine
 ↓
Rendering Engine + JavaScript Engine
 ↓
Networking
 ↓
Storage
 ↓
Screen

Networking: Fetching the Website

When you type a URL and hit Enter:

  1. Cache check – if a valid copy exists, the page may load instantly.
  2. If not, the browser performs a DNS lookup to convert twitter.com into an IP address.
  3. The browser sends an HTTP request to that IP.
  4. The server responds with HTML, CSS, JavaScript, and other assets.

Now the browser has raw files — but it still can’t display them yet.

HTML Parsing and DOM Creation

The browser parses the HTML and converts it into the DOM (Document Object Model), a tree‑like structure representing the page content.

Example HTML

<!DOCTYPE html>
<html>
  <head>
    <title>Hello</title>
  </head>
  <body>
    <h1>Hello</h1>
    <p>Welcome</p>
  </body>
</html>

DOM Tree (After Parsing)

HTML
 └── BODY
      ├── H1
      │    └── "Hello"
      └── P
           └── "Welcome"

Parsing simply means breaking the markup into meaningful pieces so the browser can understand it.

CSS Parsing and CSSOM Creation

CSS is parsed separately. The browser converts CSS into the CSSOM (CSS Object Model), which defines colors, fonts, sizes, and layout rules. While the DOM describes what elements exist, the CSSOM describes how those elements should look.

DOM + CSSOM = Render Tree

The browser combines the DOM (structure) and CSSOM (styles) to create the Render Tree, which:

  • Contains only visible elements
  • Acts as a blueprint for drawing the page

Layout (Reflow), Painting, and Display

Once the Render Tree is ready, the browser proceeds through three final stages.

1. Layout (Reflow)

Calculates the exact position, width, and height of each element, determining where everything fits on the screen.

2. Painting

Fills pixels for text, colors, images, borders, shadows, etc.

3. Display (Compositing)

Combines all painted layers and displays the final image on the screen. This is the moment you actually see the webpage.

The Complete Flow (Big Picture)

URL typed
 ↓
DNS lookup + HTTP request
 ↓
HTML → DOM
CSS → CSSOM
 ↓
DOM + CSSOM → Render Tree
 ↓
Layout (Reflow)
 ↓
Paint
 ↓
Pixels on screen

Final Thought

Don’t focus on memorizing component names. Focus on the journey:

Fetch → Understand → Organize → Draw

Every time you open a website, your browser runs this entire pipeline in milliseconds — quietly, efficiently, and repeatedly. And that’s what makes the web feel instant. 🚀

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