Semantic-UI is an HTML and CSS UI framework consisting of a themed component library and a responsive layout framework. It provides a collection of reusable interface components and a grid-based system of columns and containers designed to build responsive websites.
The framework is distinguished by its use of natural-language class naming, which maps human-readable CSS classes to specific visual styles. It also functions as a right-to-left UI toolkit, utilizing directional mirroring to adjust visual flow and element alignment for languages read from right to left.
The system covers frontend UI development and responsive web design through a modular CSS architecture. It supports custom website theming via hierarchical theme layering and uses a breakpoint-based grid to adapt layouts across different screen sizes.