Svelte is a UI framework for the web, a tool for building the parts of a web page that users see and interact with. It is developed by the Svelte contributors, full-time and part-time maintainers, and is backed by Vercel and many donors.
Svelte is a compiler. A developer writes declarative components, each combining HTML markup, a script in JavaScript or TypeScript, and CSS styles. At build time, Svelte converts these components into lean, optimized JavaScript that updates the DOM (the browser's tree of page elements) directly, so the code does minimal work in the browser. Svelte can be used for a single standalone component or for a whole application; for full-stack apps it has a companion application framework, SvelteKit. The official site offers reference documentation, an interactive tutorial and an online playground for trying code.
Svelte is free and open source software under the MIT license, and donations through Open Collective pay for project expenses such as hosting.