Quick answer is - there is currently no WebXR support on ios. If you want support you can contact apple at firstname.lastname@example.org . Or just wait until they add it
@kenchikuliu another option you could consider is Babylon React Native. This requires you to create a React Native application for iOS, but still lets you implement your 3D and augmented reality features with the Babylon.js API, and most of that code could be re-used inside a web page in a browser as well for cases where WebXR is supported. It already supports ARKit on iOS (and ARCore on Android), though it is still in an alpha or beta stage of maturity.