SunEditor is a fully-featured, vanilla JavaScript WYSIWYG HTML editor library designed for modern browsers. It provides a rich text editing experience with extensive formatting options (inline, block, lists, tables, media embedding), multiple display modes (classic, inline, balloon), and a plugin architecture that supports dropdowns, modals, commands, browsers, and input fields. The editor is built around a clean dependency injection kernel, a reducer-based event handling system for keyboard and input events, and a modular structure for UI components (toolbar, menu, controllers) and file management. It includes built-in plugins for images, videos, audio, drawing, math expressions, autocomplete, slash commands, table editing, code view, markdown preview, and document layouts (A4 page breaks). The codebase is extensively tested with unit, integration, and e2e tests (Playwright), and follows strict architectural rules enforced by dependency-cruiser. It exports TypeScript definitions, supports internationalization (20+ languages), and integrates with popular frameworks via official React/Vue wrappers. The library is distributed via npm and CDN, and demonstrates a high level of engineering craftsmanship in its event system, DOM manipulation utilities, and plugin lifecycle management.