How Browsers Render Web Pages
A deep dive into the browser rendering pipeline, from HTML parsing through the critical rendering path to paint and composite.
Understanding how browsers transform HTML, CSS, and JavaScript into pixels on screen is fundamental to building fast, responsive web applications. The rendering pipeline follows a predictable sequence—each stage has implications for performance.
The Rendering Pipeline Overview
When you navigate to a URL, the browser downloads resources and executes a multi-stage process to render the page. This pipeline consists of six main phases that work together to produce what users see.
HTML Parsing and DOM Construction
The browser reads HTML bytes and converts them into tokens via the HTML parser. These tokens are then used to build the Document Object Model (DOM)—a tree structure representing the page's structure. The DOM is built incrementally, so parsing doesn't have to complete before the next stages begin.
JavaScript can modify the DOM via document.createElement, appendChild, or by changing attributes. Script execution is blocking by default: when the parser encounters a <script> tag, it pauses HTML parsing until the script finishes. That's why placing scripts at the end of <body> or using async/defer matters for perceived performance.
CSSOM: The CSS Object Model
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