问从第一张背景图像到第二张背景图像在加载5秒后淡入

EN

body {
    background: url('path_to_your_background_image_1');
    animation: change_background 1s ease-out 5s 1 forwards;
}

@keyframes change_background {
    to {
        background-image: url('path_to_your_background_image_2');
    }
}

var timer;

function swapImage() {
  var slide = document.getElementById('slide');
  slide.className = "fade-in";
  slide.src = "http://www.w3schools.com/images/w3cert.gif";
  clearTimeout(timer);
}
timer = setTimeout("swapImage()", 5000);

@keyframes fadeIn {
to {
    opacity: 1;
  }
}

.fade-in {
  opacity: 0;
  animation: fadeIn .5s ease-in 1 forwards;
}

<img height="200" id="slide" src="http://www.w3schools.com/html/html5.gif" width="400" />