Browser Rendering
Modern browsers are sophisticated engines that transform HTML, CSS, and JavaScript into interactive web pages. This document provides an overview of the browser rendering process, based on the Frontend Roadmap (opens in a new tab).
How Browsers Work
Parsing HTML and Building the DOM
- HTML Parsing:
When a browser loads a web page, it begins by parsing the HTML document. - DOM Tree:
The HTML is converted into a Document Object Model (DOM), which represents the page structure in a tree-like format.
Parsing CSS and Building the CSSOM
- CSS Parsing:
The browser downloads and parses CSS files. - CSSOM Tree:
The CSS rules are organized into a CSS Object Model (CSSOM), which is used to apply styles to the DOM elements.
Constructing the Render Tree
- Render Tree:
The browser combines the DOM and CSSOM trees to form the render tree, containing only the nodes required for rendering. - Layout Calculation:
Using the render tree, the browser computes the layout, determining the size and position of each element.
Painting and Compositing
- Painting:
The browser paints the visual representation of elements onto the screen. - Compositing:
For complex pages, the browser may split the render tree into layers and composite them, optimizing the rendering process.
The Critical Rendering Path
- Definition:
The critical rendering path is the sequence of steps a browser follows to convert HTML, CSS, and JavaScript into pixels on the screen. - Optimization Strategies:
- Minimize render-blocking resources such as CSS and JavaScript.
- Load non-critical scripts asynchronously.
- Optimize images and other media to reduce load time.
Performance Considerations
- Reflow and Repaint:
- Reflow (Layout): Occurs when the browser recalculates the layout of the page due to DOM changes.
- Repaint: Occurs when visual changes (e.g., color changes) are applied without affecting layout.
- Minimizing Layout Thrashing:
Avoid frequent DOM manipulations that trigger reflow and repaint to maintain smooth performance. - Hardware Acceleration:
Utilize CSS transforms and animations to leverage GPU acceleration for improved rendering performance.