A real x86 PC emulated entirely in your browser. Pick an OS with a desktop + browser to surf the web from inside the VM.
Fabrice Bellard's JSLinux. Real Linux desktop (X11) on a RISC-V CPU emulated in JS. Slow boot but full GUI.
⚠️ A browser sandbox can't run VirtualBox/VMware-class VMs (no hardware passthrough, no GBs of RAM, no real network adapter). This uses v86, a JavaScript x86 emulator — same idea as VirtualBox, just slower and limited to small disk images.
For a full VirtualBox-style experience you'd need to install VirtualBox on your computer and run a real OS image there.