HTML5效果：实现树叶飘落

实现如图所示的东西效果（落叶下落）：

html代码：

<!DOCTYPE html>
<html>
<head>
<title>HTML5树叶飘落动画</title>
<meta charset="utf-8">
<meta name="viewport" content="width=500px, initial-scale=0.64">
<link rel="stylesheet" href="leaves.css" type="text/css">
<script src="leaves.js" type="text/javascript"></script>
</head>
<body>
<div id="container">
<div id="leafContainer"></div>
<div id="message">
<em>这是基于webkit的落叶动画</em>
</div>
</div>  
</body>
</html>

css代码：

body
{
background-color: #4E4226;
}
#container {
position: relative;
height: 700px;
width: 500px;
margin: 10px auto;
overflow: hidden;
border: 4px solid #5C090A;
background: #4E4226 url('images/backgroundLeaves.jpg') no-repeat top left;
}
#leafContainer {
position: absolute;
width: 100%;
height: 100%;
}
#message{
position: absolute;
top: 160px;
width: 100%;
height: 300px;
background:transparent url('images/textBackground.png') repeat-x center;
color: #5C090A;
font-size: 220%;
font-family: 'Georgia';
text-align: center;
padding: 20px 10px;
-webkit-box-sizing: border-box;
-webkit-background-size: 100% 100%;
z-index: 1;
}
em {
font-weight: bold;
font-style: normal;
}
#leafContainer > div {
position: absolute;
width: 100px;
height: 100px;
-webkit-animation-iteration-count: infinite;
-webkit-animation-direction: normal;
-webkit-animation-timing-function: linear;
}
#leafContainer > div > img {
position: absolute;
width: 100px;
height: 100px;
-webkit-animation-iteration-count: infinite;
-webkit-animation-direction: alternate;
-webkit-animation-timing-function: ease-in-out;
-webkit-transform-origin: 50% -100%;
}
@-webkit-keyframes fade
{
0%   { opacity: 1; }
95%  { opacity: 1; }
100% { opacity: 0; }
}
@-webkit-keyframes drop
{
0%   { -webkit-transform: translate(0px, -50px); }
100% { -webkit-transform: translate(0px, 650px); }
}
@-webkit-keyframes clockwiseSpin
{
0%   { -webkit-transform: rotate(-50deg); }
100% { -webkit-transform: rotate(50deg); }
}
@-webkit-keyframes counterclockwiseSpinAndFlip 
{
0%   { -webkit-transform: scale(-1, 1) rotate(50deg); }
100% { -webkit-transform: scale(-1, 1) rotate(-50deg); }
}

js代码：

const NUMBER_OF_LEAVES = 30;
function init(){
var container = document.getElementById('leafContainer');
for (var i = 0; i < NUMBER_OF_LEAVES; i  ) {
container.appendChild(createALeaf());
}
}
function randomInteger(low, high){
return low   Math.floor(Math.random() * (high - low));
}
function randomFloat(low, high){
return low   Math.random() * (high - low);
}
function pixelValue(value){
return value   'px';
}
function durationValue(value){
return value   's';
}
function createALeaf(){
var leafDiv = document.createElement('div');
leafDiv.style.top = "-100px";
leafDiv.style.left = pixelValue(randomInteger(0, 500));
leafDiv.style.webkitAnimationName = 'fade, drop';
var fadeAndDropDuration = durationValue(randomFloat(5, 11));
leafDiv.style.webkitAnimationDuration = fadeAndDropDuration   ', '   fadeAndDropDuration;
var leafDelay = durationValue(randomFloat(0, 5));
leafDiv.style.webkitAnimationDelay = leafDelay   ', '   leafDelay;
var image = document.createElement('img');
image.src = 'images/realLeaf'   randomInteger(1, 5)   '.png';
var spinAnimationName = (Math.random() < 0.5) ? 'clockwiseSpin' : 'counterclockwiseSpinAndFlip';
image.style.webkitAnimationName = spinAnimationName;
var spinDuration = durationValue(randomFloat(4, 8));
image.style.webkitAnimationDuration = spinDuration;
leafDiv.appendChild(image);
return leafDiv;
}
window.addEventListener('load', init, false);