An "Extended Kernel" is a community-made modification (specifically, a .dll replacement) that injects modern Windows 10 and Windows 11 API calls directly into the older Windows 8.1 kernel.
Recent versions of Chromium (Google Chrome, Microsoft Edge) and Mozilla Firefox. Windows 8.1 Extended Kernel
As of early 2026, there is project for Windows 8.1 in the same vein as the well-known Windows Vista Extended Kernel . Windows 8.1 Extended Kernel