Unreal Engine 3, the game engine that runs fan favorites like Mass Effect, Gears of War, and Infinity Blade, has been ported to run inside a web browser without any plug-ins. Using JavaScript, specifically the asm.js sublanguage and Emscripten compiler, this engine can run on almost any modern web browser. While this is just a tech demo, it goes to show how the web browser is starting to become a completely valid native platform for games.

In an impressive feat, engineers at both Epic Games and Mozilla were able to port Unreal Engine 3 to JavaScript and WebGL in only four days. In this minuscule time frame, they were able to get the famous Epic Citadel demo in a working state to show off at the Game Developer Conference (GDC) this week. While the performance isn’t quite as good as running the engine using straight C++, it does get surprisingly close. If your browser is optimized for asm.js, like Firefox Nightly is, it will run at roughly half the speed of the native C++ engine. (See: WebGL is fundamentally flawed.)

At the end of the video, we’re instructed to visit Emscripten or Mozilla Hacks to find out more. As of the time this article was written, neither of those sites have been updated with information about this project. Worse yet, we’re not going to get to see this demo for ourselves for a few more weeks. That said, Epic Citadel is already available running in Flash. If you absolutely have to see what Unreal Engine 3 looks like in a browser, you can experience it right this second. Don’t come crying to me when your browser crashes or your fans kick into overdrive, though.

As clever as this port is, it isn’t apparent if Epic Games even intends to license out the engine for web development. For all we know, this was just a cute side project to show off for the GDC crowd. Even so, it’s really impressive what modern web browsers are capable of. Take a look at Google — it’s working hard to get games to come to web browsers with its Native Client project. We’re already seeing great indie games like Bastion on the Chrome Web Store. Regardless of the implementation, we’re getting closer to the write-once, run-everywhere dream. While top-tier games won’t be coming to the browser any time soon, we can expect the complexity of web games to grow rapidly. It’s not just Bejeweled and poker anymore.

Now read: Real-time tech demos that showcase the future of console and PC gaming