This feature adds a new optional capability to WebGPU that exposes a new WGSL shader builtin, 'primitive_index'. This builtin provides a per-primitive index to fragment shaders on supported hardware, similar to the existing vertex_index and instance_index builtins. The primitive index is useful for advanced graphical techniques, such as virtualized geometry.
Explainers: https://github.com/gpuweb/gpuweb/blob/main/proposals/primitive-index.md