This little web app demonstrates how the electric and magnetic fields transform under a Lorentz boost. It can also display the Poynting vector and some dynamical quantities associated with a charged particle co-located with the field-vectors.
I made this with TypeScript, Next.js/React, zustand, Tailwind CSS, three.js/react-three-fiber, MathJax/better-react-mathjax, and React Hotkeys Hook. It's hosted on Vercel.