An effective progressive web app design focuses on key principles like responsiveness, fast loading, offline functionality, and seamless user experience. PWAs should be lightweight, mobile-first, and optimized for different screen sizes. Implementing service workers ensures smooth performance, even with poor connectivity. A well-structured UI with intuitive navigation enhances usability. Security via HTTPS and push notifications improve trust and engagement. Optimizing assets and caching strategies help maintain speed. Lastly, ensuring cross-browser compatibility and easy installation enhances accessibility. By following these design considerations, a progressive web app delivers a high-performance, app-like experience across all devices.