An introduction to Wikipedia's ogv.js media compatibility shim, which uses WebAssembly codecs to provide video file format compatibility for VP9 and AV1 video in browsers that don't play them natively. Will explore the division between the JS and Wasm parts of the code base, and how advances in emscripten and LLVM create opportunities and challenges for performance as different browsers implement different levels of the spec (threading, SIMD, etc).