WebVR is a standard that browsers should follow when implementing support.
For the browsers that don’t support it natively, there is something that is none as a polyfill which is a software library that implements the logic of the standard while support in progress hence the name.
It allows developers to use future functionality in the present and is a common practise in the web development space.
At the moment, not all browsers support WebVR natively or are disabled behind browser feature flags (as shown here) so the polyfill is needed.
Unfortunately, the WebVR polyfill that PlayCanvas is outdated compared to the Git master that it was forked from.
The biggest bug is that Chrome also changed the units that rotation of the display is returned as from/to radians/degrees.
When PlayCanvas is used to make a WebVR app, the polyfill gets included.
I think after the build is exported and the updated WebVR polyfill is replaced, PlayCanvas does work with WebVR as well it can be.
When it was first launched, WebVR Labs supported these platforms. I had it working on DayDream, GearVR, cardboard and Rift on PC. Not sure if iOS worked then