Leptos is a web framework for writing full-stack applications in Rust. Components are written once and can run server-side or client-side (WebAssembly). This isomorphic model eliminates code duplication and enables progressive enhancement (page works without JavaScript, enhanced with it). Leptos emphasizes safety (Rust prevents null pointer errors, thread unsafety), performance (compiled language), and ergonomics (reactive component model similar to React/Svelte).