CSS后台位置无法在Mobile Safari(iPhone/iPad)中运行

@media only screen and (max-device-width: 768px) {
  /* 在iPad上应用特定的样式 */
  .background {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
  }
}

@media only screen and (max-device-width: 480px) {
  /* 在iPhone上应用特定的样式 */
  .background {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
  }
}

window.onload = function() {
  var isMobileSafari = navigator.userAgent.match(/(iPod|iPhone|iPad)/) && navigator.userAgent.match(/AppleWebKit/);
  if (isMobileSafari) {
    var background = document.getElementById('background');
    background.style.position = 'fixed';
    background.style.top = '0';
    background.style.left = '0';
    background.style.width = '100%';
    background.style.height = '100%';
  }
};