XRWebGLBinding.getViewSubImage()

getViewSubImage(layer, view)

const xrGlBinding = new XRWebGLBinding(xrSession, gl);
const layer = xrGlBinding.createProjectionLayer({});
const framebuffer = gl.createFramebuffer();

xrSession.updateRenderState({ layers: [layer] });
xrSession.requestAnimationFrame(onXRFrame);

function onXRFrame(time, xrFrame) {
  xrSession.requestAnimationFrame(onXRFrame);

  gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);

  for (let view in xrViewerPose.views) {
    let subImage = xrGlBinding.getViewSubImage(layer, view);
    gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0,
      gl.TEXTURE_2D, subImage.colorTexture, 0);
    gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT,
      gl.TEXTURE_2D, subImage.depthStencilTexture, 0);
    let viewport = subImage.viewport;
    gl.viewport(viewport.x, viewport.y, viewport.width, viewport.height);

    // Render from the viewpoint of xrView
  }
}