<div></div>

    <!-- 样式 -->
    <style>
    
        /*定义动画*/
        @keyframes myfirst
        {
            0%   {background: red;width: 20%;}
            25%  {background: yellow;}
            50%  {background: blue;}
            100% {background: green;width: 100%;}
        }
        
        div
        {
            width: 200px;
            height: 200px;
            background-color: #999;
            
            /*执行动画*/
            animation: myfirst 5s;
        }
        
    </style>