Built from the FireEmblemUniverse/fireemblem8u decompilation, running on a WebAssembly build of the mGBA core.
Your ROM cannot leave this device. It is read into memory by the browser's own file reader and kept in this page's local storage so you need not pick it again. This page is static files with no server component, and its Content Security Policy forbids every kind of network request — so there is nowhere for the bytes to go.