There's already an API on the web for Lighting Estimation (https://w3c.github.io/ambient-light/) but it currently only supports getting the ambient intensity.
It would be useful for improving the realism of AR rendering to have slightly more information than that. For example the color temperature would be helpful and is generally available on the underlying XR platforms that would back WebXR. There are also other more advanced lighting options available on some platforms that would be worth investigating as optional features. One example is this from ARKit: https://developer.apple.com/documentation/arkit/arenvironmentprobeanchor