Creates browser-based AR/VR/XR experiences using the WebXR Device API, builds 3D scenes with Three.js, A-Frame, and Babylon.js, implements hand tracking, raycasting, hit testing, spatial anchors, and controller input, and applies performance optimizations like LOD, occlusion culling, and shader tuning for headsets and mobile AR. Use when the user asks about WebXR, virtual reality, augmented reality, immersive 3D web apps, VR headsets, spatial computing, or mentions frameworks like Three.js, A-Frame, Babylon.js, or file formats like .glb or .gltf.