LibWeb

LibWeb is SerenityOS’s comprehensive web engine, implementing modern web standards including HTML5, CSS3, JavaScript, and Web APIs. It provides a full-featured browser engine with standards-compliant DOM, rendering, and scripting capabilities.

Architecture Overview

LibWeb is organized into subsystems that mirror web specifications:

DOM: Document Object Model and tree structures
HTML: HTML parsing, elements, and semantics
CSS: Style computation and cascade
Layout: Box model and positioning
Painting: Rendering pipeline
JavaScript: Integration with LibJS
Fetch: Network resource loading
Web APIs: Canvas, WebGL, Storage, and more

Core Subsystems

DOM (Document Object Model)

Document

class

The root document object representing an HTML or XML document.

class Document : public ParentNode,
                 public NonElementParentNode<Document>,
                 public HTML::GlobalEventHandlers {
    // Document lifecycle
    HTML::DocumentReadyState ready_state() const;
    
    // DOM tree access
    Element* document_element();
    Element* body();
    Element* head();
    
    // Element creation
    NonnullRefPtr<Element> create_element(String const& tag_name);
    NonnullRefPtr<Text> create_text_node(String const& data);
    
    // Querying
    Element* get_element_by_id(String const& id);
    Vector<Element*> get_elements_by_tag_name(String const&);
};

Location: LibWeb/DOM/Document.h

Node Types

LibWeb implements the complete DOM node hierarchy:

Node: Base class for all DOM nodes
Element: Elements with attributes and tag names
Text: Text content nodes
Comment: Comment nodes
DocumentType: DOCTYPE declarations
DocumentFragment: Lightweight document containers
Attr: Attribute nodes

Node Operations:

void append_child(Node&);
void remove_child(Node&);
void insert_before(Node& node, Node& child);
Node* parent_node();
Node* first_child();
Node* last_child();
Node* next_sibling();
Node* previous_sibling();

HTML Elements

HTMLElement

class

Base class for all HTML elements.

class HTMLElement : public DOM::Element,
                    public HTML::GlobalEventHandlers {
    // Content properties
    String inner_text();
    void set_inner_text(StringView);
    String outer_text();
    
    // Layout properties
    int offset_top() const;
    int offset_left() const;
    int offset_width() const;
    int offset_height() const;
    Element* offset_parent() const;
    
    // Focus and interaction
    void focus();
    void blur();
    void click();
    
    // Editability
    bool is_editable() const;
    bool is_content_editable() const;
};

Location: LibWeb/HTML/HTMLElement.h

Common HTML Elements:

Layout: HTMLDivElement, HTMLSpanElement
Forms: HTMLFormElement, HTMLInputElement, HTMLButtonElement
Media: HTMLImageElement, HTMLVideoElement, HTMLAudioElement
Semantic: HTMLHeadingElement, HTMLParagraphElement
Tables: HTMLTableElement, HTMLTableRowElement
Canvas: HTMLCanvasElement

CSS (Cascading Style Sheets)

CSS Engine

LibWeb implements a complete CSS3 engine:Subsystems:

Parser: CSS syntax parsing
Selectors: Complex selector matching
Cascade: Specificity and inheritance
StyleCompute: Property computation
Values: All CSS value types
Properties: Individual property handlers

Major Features:

Flexbox and Grid layouts
CSS animations and transitions
CSS transforms (2D and 3D)
Media queries
Custom properties (CSS variables)
Pseudo-classes and pseudo-elements

Layout Engine

LibWeb’s layout engine implements the CSS box model and positioning schemes.

Layout Nodes

class Layout::Node {
    virtual void compute_width();
    virtual void compute_height();
    virtual void layout(LayoutMode);
    
    Box const& box_model() const;
    CSSPixelRect absolute_rect() const;
};

Layout Types:

BlockContainer: Block formatting context
InlineNode: Inline content
FlexFormattingContext: Flexbox layout
GridFormattingContext: CSS Grid layout
TableBox: Table layout
SVGBox: SVG element layout

JavaScript Integration

Bindings

namespace

Automatic bindings between C++ and JavaScript using Web IDL.LibWeb uses .idl files to generate JavaScript bindings:

interface Document : Node {
    readonly attribute DOMString URL;
    readonly attribute Element? documentElement;
    Element? getElementById(DOMString elementId);
};

The IDL compiler generates wrapper classes for JavaScript access.Location: LibWeb/Bindings/

Web APIs

Implemented Web APIs:

Canvas 2D: 2D drawing context
WebGL: 3D graphics rendering
Web Storage: localStorage and sessionStorage
Fetch API: Modern HTTP requests
WebSockets: Bidirectional communication
Web Animations: Animation API
Intersection Observer: Viewport observation
Mutation Observer: DOM change observation
File API: File and Blob handling
Crypto API: Web Cryptography

Major Subsystems

ARIA (Accessibility)

ARIA

namespace

Accessible Rich Internet Applications support.

namespace Web::ARIA {
    enum class Role {
        Button, Checkbox, Link, Heading,
        Dialog, Menu, Navigation, // ...
    };
    
    // ARIA properties and states
    // aria-label, aria-hidden, aria-expanded, etc.
}

Location: LibWeb/ARIA/

Animations

Web Animations API

class Animation {
    void play();
    void pause();
    void cancel();
    void finish();
    
    double currentTime() const;
    void set_currentTime(double);
    
    AnimationPlayState playState() const;
};

class KeyframeEffect {
    void set_target(Element*);
    void set_keyframes(Vector<Keyframe>);
};

Location: LibWeb/Animations/

CSS Subsystems

CSS Implementation

Core Components:

CSSStyleSheet: Stylesheet representation
CSSRule: Style rules (selectors + declarations)
CSSStyleDeclaration: Property declarations
StyleResolver: Matching and cascade
StyleComputer: Computed value generation

Property System:

Length values (px, em, rem, %, vh, vw)
Colors (named, hex, rgb(), hsl(), etc.)
Transforms and animations
Filters and effects

Location: LibWeb/CSS/

Fetch API

Fetch

namespace

Modern HTTP request API.

class Request {
    String const& url() const;
    String const& method() const;
    Headers const& headers() const;
};

class Response {
    u16 status() const;
    String status_text() const;
    Headers const& headers() const;
    
    Promise<ByteBuffer> array_buffer();
    Promise<String> text();
    Promise<JsonValue> json();
};

Location: LibWeb/Fetch/

WebGL

WebGL Support

LibWeb provides WebGL 1.0 support through LibGL:

class WebGLRenderingContext {
    // OpenGL ES 2.0 compatible API
    void clear(GLbitfield mask);
    void clearColor(GLfloat r, GLfloat g, GLfloat b, GLfloat a);
    
    WebGLProgram createProgram();
    WebGLShader createShader(GLenum type);
    
    void drawArrays(GLenum mode, GLint first, GLsizei count);
    void drawElements(GLenum mode, GLsizei count, GLenum type, GLintptr offset);
};

WebGL calls are translated to LibGL OpenGL operations.

Parsing and Tokenization

HTML Parser

HTML5 Parser

LibWeb implements the HTML5 parsing algorithm:

Tokenization: Character stream to tokens
Tree Construction: Token stream to DOM
Error Recovery: Robust error handling
Foreign Content: SVG and MathML integration

The parser handles:

Document fragments
Script execution during parsing
Character encodings
Quirks mode detection

Location: LibWeb/HTML/Parser/

CSS Parser

Robust CSS3 parsing with error recovery and vendor prefix support.

Event System

EventTarget

class

Base class for objects that can receive events.

class EventTarget {
    void addEventListener(String const& type, 
                         EventListener*, 
                         bool use_capture = false);
    
    void removeEventListener(String const& type,
                            EventListener*,
                            bool use_capture = false);
    
    bool dispatchEvent(Event&);
};

Event Types

Mouse Events:

click, dblclick
mousedown, mouseup, mousemove
mouseenter, mouseleave

Keyboard Events:

keydown, keyup, keypress

Form Events:

submit, change, input, focus, blur

Document Events:

DOMContentLoaded, load, unload

Custom Events:

User-defined event types

Rendering Pipeline

Paint System

The rendering pipeline transforms layout to pixels:

Layout: Compute positions and sizes
Stacking Context: Z-order determination
Paint: Generate display list
Composite: Layer composition

class Paintable {
    virtual void paint(PaintContext&) const;
    virtual HitTestResult hit_test(CSSPixelPoint) const;
};

Location: LibWeb/Painting/

Storage APIs

Storage

class

Web Storage (localStorage/sessionStorage) implementation.

class Storage {
    size_t length() const;
    String key(size_t index) const;
    String get_item(String const& key) const;
    void set_item(String const& key, String const& value);
    void remove_item(String const& key);
    void clear();
};

Location: LibWeb/HTML/Storage.h

SVG Support

Scalable Vector Graphics rendering integrated into the layout engine.

SVG Features:

Basic shapes (rect, circle, ellipse, line, polygon)
Paths with full path data support
Text rendering
Gradients and patterns
Transformations
Clipping and masking

Location: LibWeb/SVG/

Encoding and URLs

URL

class

URL parsing and manipulation.

class URL {
    String scheme() const;
    String host() const;
    u16 port() const;
    String path() const;
    String query() const;
    String fragment() const;
    
    static URL parse(String const&);
};

Location: LibWeb/DOMURL/

Cookie API

class Cookie {
    String name;
    String value;
    String domain;
    String path;
    Core::DateTime expiry_time;
    bool secure;
    bool http_only;
    SameSite same_site;
};

Full cookie jar with domain matching and security policies.Location: LibWeb/Cookie/

FileAPI

File

class

File and Blob handling for file uploads and processing.

class Blob {
    u64 size() const;
    String type() const;
    Promise<String> text();
    Promise<ByteBuffer> array_buffer();
};

class File : public Blob {
    String name() const;
    i64 last_modified() const;
};

Location: LibWeb/FileAPI/

Performance and Timing

Performance APIs

Performance API: Navigation and resource timing
requestAnimationFrame: Animation scheduling
Intersection Observer: Viewport-based loading
Resize Observer: Element size changes

Used for performance monitoring and efficient rendering.

Usage Example

// Create a document
auto document = DOM::Document::create();

// Create elements
auto body = document->create_element("body");
auto div = document->create_element("div");
auto text = document->create_text_node("Hello, World!");

// Build DOM tree
div->append_child(text);
body->append_child(div);
document->append_child(body);

// Set attributes
div->set_attribute("class", "greeting");
div->set_attribute("id", "main");

// Query elements
auto element = document->get_element_by_id("main");

// Style computation and layout
document->update_style();
document->update_layout();

// Render to bitmap
auto bitmap = document->paint_to_bitmap();

Source Location

Directory: Userland/Libraries/LibWeb/ Major Subdirectories:

DOM/: Document Object Model
HTML/: HTML elements and parsing
CSS/: Style system
Layout/: Layout engine
Painting/: Rendering
Bindings/: JavaScript integration
Fetch/: Network requests
WebGL/: 3D graphics
SVG/: Vector graphics
ARIA/: Accessibility
Animations/: Web Animations API

Core Libraries

System Libraries

Architecture Overview

Core Subsystems

DOM (Document Object Model)

HTML Elements

CSS (Cascading Style Sheets)

Layout Engine

JavaScript Integration

Web APIs

Major Subsystems

ARIA (Accessibility)

Animations

CSS Subsystems

Fetch API

WebGL

Parsing and Tokenization

HTML Parser

CSS Parser

Event System

Rendering Pipeline

Storage APIs

SVG Support

Encoding and URLs

FileAPI

Performance and Timing

Usage Example

Source Location

Build docs developers (and LLMs) love

Core Libraries

System Libraries

Documentation Index

​Architecture Overview

​Core Subsystems

​DOM (Document Object Model)

​HTML Elements

​CSS (Cascading Style Sheets)

​Layout Engine

​JavaScript Integration

​Web APIs

​Major Subsystems

​ARIA (Accessibility)

​Animations

​CSS Subsystems

​Fetch API

​WebGL

​Parsing and Tokenization

​HTML Parser

​CSS Parser

​Event System

​Rendering Pipeline

​Storage APIs

​SVG Support

​Encoding and URLs

​Cookie Management

​FileAPI

​Performance and Timing

​Usage Example

​Source Location

Build docs developers (and LLMs) love

Architecture Overview

Core Subsystems

DOM (Document Object Model)

HTML Elements

CSS (Cascading Style Sheets)

Layout Engine

JavaScript Integration

Web APIs

Major Subsystems

ARIA (Accessibility)

Animations

CSS Subsystems

Fetch API

WebGL

Parsing and Tokenization

HTML Parser

CSS Parser

Event System

Rendering Pipeline

Storage APIs

SVG Support

Encoding and URLs

Cookie Management

FileAPI

Performance and Timing

Usage Example

Source Location