As a follow up to creating WebXR cameras, the fact that the distance a camera moves as you walk is measured in meters, means all your geometry needs to makes sense in meters.
This can create issues in Blender. If you create or re-scale geometry, you might want to zoom in while working on it. Blender has settings for the viewport to control how close you may come before it is clipped. This is separate from what any of the scene cameras have. View settings are in the Transforms panel, but in Blender 2.80 are not done the same way.
Interestingly, most will probably set the minZ to 0.1 meters for a WebXRCamera, so after editing, setting this back might be a quick an dirty way to see if your detail is too high or too low.
Also, the exporter has settings for the number of decimals of the various geometry types. The positions value might need to be changed.