Make a tiny browser engine from scratch

This article details building a miniature browser rendering engine in JavaScript, offering a practical approach to understanding browser rendering principles. The author, a seasoned frontend developer, eschews a full-scale engine for a more manageable, educational project. Inspired by the Rust-based Robinson engine, this JavaScript version is available on GitHub as tiny-rendering-engine.

The rendering process is broken down into five key phases, visually represented in a flowchart:

These phases are further detailed in separate GitHub branches:

1. HTML Parser (v1): Converts HTML into a DOM tree.
2. CSS Parser (v2): Transforms CSS into a collection of CSS rules.
3. Style Tree Builder (v3): Combines the DOM and CSS rules to create a style tree.
4. Layout Tree Builder (v4): Generates a layout tree from the style tree, incorporating the CSS box model.
5. Painter (v5): Renders the layout tree onto the canvas using the node-canvas library.

The article then dives into the specifics of the HTML parser, outlining its simplified design with restrictions to streamline the process. The core logic is explained, covering methods like parse(), parseElement(), parseText(), parseTag(), and parseAttrs(), along with helper functions for space removal and text slicing. Node types (Element and Text) are defined using TypeScript interfaces.

The CSS parser, described next, is comparatively simpler, building upon the concepts introduced in the HTML parser section. It handles selectors (tag names, IDs, classes) and declarations, creating a structured representation of CSS rules.

The Style Tree Builder phase merges the DOM and CSS rule collections. The process involves traversing the DOM, matching selectors against CSS rules, and creating StyleNode objects that link DOM nodes with their corresponding styles. Inheritance of properties (like color and font-size) from parent nodes is addressed. Inline styles are also handled, overriding other styles where applicable.

The Layout Tree Builder phase is the most complex, focusing on the CSS box model and layout algorithms. Block and inline layouts are supported, with anonymous block containers used to manage mixed layouts. The algorithm calculates node dimensions and positions, traversing the tree top-down for width and bottom-up for height calculations. The article provides a detailed explanation of width calculation, addressing scenarios with auto values and margin adjustments. Position calculation and child node traversal are also explained.

Finally, the Painter phase uses the node-canvas library to render the layout tree visually. The article touches upon the fundamental principles of rasterization, illustrating how lines and characters are rendered using pixel manipulation. The rendering process iterates through the layout tree, painting backgrounds, borders, and text for each node.

The article concludes with a rendered example image, demonstrating the engine's output, and a list of reference materials. The entire project, while simplified, provides a valuable learning experience for understanding the complexities of browser rendering.

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