为flask中的HTML加载页面时滚动球

<div id="loading-spinner"></div>

#loading-spinner {
  width: 40px;
  height: 40px;
  border: 4px solid #f3f3f3;
  border-top: 4px solid #3498db;
  border-radius: 50%;
  animation: spin 2s linear infinite;
}

@keyframes spin {
  0% { transform: rotate(0deg); }
  100% { transform: rotate(360deg); }
}

window.addEventListener('DOMContentLoaded', function() {
  var spinner = document.getElementById('loading-spinner');
  spinner.style.display = 'block';
});