WebXR and Quest: How to enable fixed foveated rendering (FFR)?

Which requiredFeatures or optionalFeatures does the Oculus browser support on Quest (1 or 2)?
From this Stackoverflow-post: https://stackoverflow.com/questions/61291789/how-to-set-highrefreshrate-in-webxr 

we get some suggestions about features that can be enabled/changed:

• Foveation: no-fixed-foveation, low-fixed-foveation-level, medium-fixed-foveation-level, high-fixed-foveation-level
• Refresh rate: low-refresh-rate, high-refresh-rate

Still, when trying this out, only refresh rate works:

navigator.xr.requestSession('immersive-vr', {
                    requiredFeatures: ['local-floor'],
                    optionalFeatures: ['low-refresh-rate']
                })

The above will get me lower refresh rate so that one works!!

However,

navigator.xr.requestSession('immersive-vr', {
                    requiredFeatures: ['local-floor'],
                    optionalFeatures: ['high-fixed-foveation-level']
                })

...has no effect...

Do we have any official documentation on this??? Or, more simply, how do I enable FFR on WebXR/Quest?
Best,
Mikael