code: func (p *WebPage) ViewportSize() (width, height int, err error) {
  	var resp struct {
  		Width  int `json:"width"`
  		Height int `json:"height"`
  	}
  	if err := p.ref.process.doJSON("POST", "/webpage/ViewportSize", map[string]interface{}{"ref": p.ref.id}, &resp); err != nil {
  		return 0, 0, err
  	}
  	return resp.Width, resp.Height, nil
  }
docs: // ViewportSize returns the size of the viewport on the browser.