PointerLock API introduce movementX/Y that provides data about relative mouse movements. (https://w3c.github.io/pointerlock/#extensions-to-the-mouseevent-interface)

However there are some interop issues with the coordinate space:
1. Chrome and Edge expose movementX/Y in physical pixels, other major browsers are different.
2. CSSOM View Module spec (https://drafts.csswg.org/cssom-view/#dom-mouseevent-screenx) defines screenX/Y’s coordinate space as CSS pixels; however, most major browsers expose them in device-independent pixels.
3. There is no way developers can convert between physical pixels and device-independent pixels (neither in spec nor in implementations).

See the design doc for details: https://docs.google.com/document/d/1mYk4qMxBVsFweqFOku2FZvMajejp2Q7XWGIld9ivlXE/edit?ts=5c1bd1fa#