Howdy!
AR is supported. But not all AR features. At the moment, you can enter an immersive-ar session and present elements. There are a few limitations to the current implementation, which will change soon:
- Pointer down and up is not triggered. So you cannot click buttons ATM.
- Plane detection and anchors are not yet supported.
So what you have is your estimated height. The Y axis is your device’s height, and 0 should be the floor (if your device detected it correctly).
To enable AR (working on a simpler implementation for that) use this code:
var xr = await scene.createDefaultXRExperienceAsync({/* options, if you want */})
// default is vr, change to ar
xr.enterExitUI["_buttons"][0].sessionMode = "immersive-ar";
This will work in any ar-enabled browser.
A small tip - make sure there is no environment set 
I will soon start working on fully implementing the AR specs so you can interact with the entire scene.