v86 - x86 emulator running entirely in web browsers

Unlike other browser-based x86 emulators, OSes running inside v86 can actually access internet via a transparent proxy relay. You can load your own OS images, or choose one from a pretty comprehensive list (Arch Linux, Windows 1.01 to 2000, SerenityOS, *BSD, Android 1.6, Haiku, QNX, and so on).

The VM images are loaded in stages, so it boots fast. When you run a program or code that's not loaded yet, it'll fetch the image and perform a JIT compilation of x86 code to wasm. The delay when fetching additional images and performing JIT compilation is noticeable, but the program run fast afterward considering this is a full x86 emulator running entirely within a web browser.