Open saved html file in an virtual browser and save a screen shot to PNG format.

def save_as_png(self, filename, width=300, height=250, render_time=1):
        """Open saved html file in an virtual browser and save a screen shot to PNG format."""
        self.driver.set_window_size(width, height)
        self.driver.get('file://{path}/{filename}'.format(
            path=os.getcwd(), filename=filename + ".html"))
        time.sleep(render_time)
        self.driver.save_screenshot(filename + ".png")