Once UI

Once UI is an open-source design system specifically built for Next.js applications. It provides developers with semantic layout components like Row, Column, and Grid that create human-readable code while maintaining machine-writable syntax. This approach helps reduce code volume by up to 70% compared to traditional CSS frameworks, making it easier to build and maintain complex interfaces.

The system includes over 100 pre-styled components that are fully customizable through a centralized theme configuration. All styling and theming is managed in a single file, eliminating the chaos of scattered CSS files and design tokens. The components are optimized for Next.js, ensuring fast performance and seamless integration with the framework's features. The syntax is designed to work well with AI coding assistants, preventing the 'hallucination' of obscure code that can occur with less structured approaches.

Once UI is ideal for frontend developers and indie creators who want to build web applications quickly without sacrificing code quality or design consistency. It's particularly useful for teams maintaining multiple projects who need a unified design language, and for developers working with AI assistants who benefit from the predictable, semantic structure. The system helps create professional-looking interfaces while keeping the codebase clean and maintainable.