The same source code what native (ie, Windows, or Linux executable) version of Xemu is built from, just built with a compiler named "emscripten", which makes it possible to generatre "web-aware" result (WebAssembly, asm.js). This is a highly work in progress version, you can see ugly, and partially broken user interface (among other problems), and it requires fairly modern browser, possibly (latest version of) Firefox or Chrome (even Safari is too old, it seems, no hope for IE too much, of course, or anything MS ... Maybe Edge with the Blink rendering engine which is not even released yet officially ..). Beware, it causes quite huge data download when visiting this page, around 20Mbyte or such in case of Mega65 emulator for example. This version also includes a mostly working setup to really see Xemu/Mega65 in action (prepared SD-card image, etc).
Please use the new page to download Xemu: https://github.lgb.hu/xemu/