WebXR is available to test on the Safari browser that comes with Vision Pro Simulator. Instructions to enable it are available here.

VisionOS comes with a Safari that supports WebXR. You can test in on the Vision Pro Simulator. Sadly, as you're figuring out, a version of Safari with WebXR support will only be available for Vision Pro and not iOS devices. This is discussed in greater length here: https://www.lowpass.cc/p/apple-vision-pro-webxr-standard-ios.