The read-only recommendedViewportScale property of the XRView interface is the recommended viewport scale value that you can use for XRView.requestViewportScale() if the user agent has such a recommendation; null otherwise.

A number greater than 0.0 and less than or equal to 1.0; or null if the user agent does not provide a recommended scale.

Dynamic viewport scaling allows applications to only use a subset of the available framebuffer. The feature may not be available on all systems since it depends on driver support, so you might want to ensure that XRView.requestViewportScale exists before calling it.

for (const view of pose.views) {
  if (view.requestViewportScale) {
    view.requestViewportScale(view.recommendedViewportScale);
  }
  const viewport = glLayer.getViewport(view);
}

• XRView.requestViewportScale()