The TsilaAllaoui/warioland4 decompilation, built to a byte-identical ROM and run here on a WebAssembly build of mGBA.
Your ROM stays on this device. It is read into memory with the browser's own file reader and kept in this page's local storage so you don't have to pick it again. There is no server to upload it to — this page is static files, and its Content-Security-Policy forbids any connection to another origin.