如何通过纯CSS实现网页的平滑滚动背景淡入

<div class="container">
  <div class="bg"></div>
  <!-- 页面内容 -->
  ...
</div>

.container {
  position: relative;
  overflow: hidden;
  width: 100%;
  height: 600px;
}

.bg {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-image: url(背景图片的路径);
  background-position: center;
  background-size: cover;
  opacity: 0;
  z-index: -1;
  transition: opacity 0.5s ease;  
}

window.addEventListener("scroll", function() {
  var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
  var scrollHeight = document.documentElement.scrollHeight || document.body.scrollHeight;
  var clientHeight = document.documentElement.clientHeight || document.body.clientHeight;
  var scrollPercent = (scrollTop / (scrollHeight - clientHeight)) * 100;
  
  var bg = document.querySelector(".bg");
  bg.style.opacity = scrollPercent / 100;
});