How Your Browser Renders Webpages: Parsing HTML with Trees and Graphs

Introduction

When you type a URL into your browser and hit enter, a lot happens in the blink of an eye. Your browser doesn’t just magically display a webpage — it processes, parses, and renders data using some of the most efficient algorithms and data structures. At the heart of this process are trees and graphs, which help browsers convert raw HTML, CSS, and JavaScript into a beautiful, interactive webpage.

In this article, we’ll take a deep dive into the step-by-step process of webpage rendering, focusing on the underlying data structures like the DOM tree, CSSOM tree, and rendering tree.

The Rendering Process: An Overview

Here’s the high-level flow of how a browser renders a webpage:

1. HTML Parsing: The browser parses the HTML file and constructs a DOM tree (Document Object Model).
2. CSS Parsing: Simultaneously, CSS files are parsed into a CSSOM tree (CSS Object Model).
3. JavaScript Execution: JavaScript may modify the DOM and CSSOM trees dynamically.
4. Rendering Tree Construction: The browser combines the DOM and CSSOM trees into a rendering tree.
5. Layout: The rendering tree is used to calculate the…