CSS3动画产生圆圈由小变大向外扩散的效果

涉及到 CSS3 的动画（animation）、2D 转换（transform: scale），具体如代码所示。
github： https://github.com/wind-stone/CSS3-Circle-Diffusion
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
@keyframes warn {
    0% {
        transform: scale(0);
        opacity: 0.0;
    }
    25% {
        transform: scale(0);
        opacity: 0.1;
    }
    50% {
        transform: scale(0.1);
        opacity: 0.3;
    }
    75% {
        transform: scale(0.5);
        opacity: 0.5;
    }
    100% {
        transform: scale(1);
        opacity: 0.0;
    }
}
@-webkit-keyframes "warn" {
    0% {
        -webkit-transform: scale(0);
        opacity: 0.0;
    }
    25% {
        -webkit-transform: scale(0);
        opacity: 0.1;
    }
    50% {
        -webkit-transform: scale(0.1);
        opacity: 0.3;
    }
    75% {
        -webkit-transform: scale(0.5);
        opacity: 0.5;
    }
    100% {
        -webkit-transform: scale(1);
        opacity: 0.0;
    }
}

.container {
    position: relative;
    width: 40px;
    height: 40px;
    border: 1px solid #000;
}
/* 保持大小不变的小圆圈  */
.dot {
    position: absolute;
    width: 6px;
    height: 6px;
    left: 15px;
    top: 15px;
    -webkit-border-radius: 20px;
    -moz-border-radius: 20px;
    border: 2px solid red;
    border-radius: 20px;
    z-index: 2;
}
/* 产生动画（向外扩散变大）的圆圈  */
.pulse {
    position: absolute;
    width: 24px; 
    height: 24px;
    left: 2px;
    top: 2px;
    border: 6px solid red;
    -webkit-border-radius: 30px;
    -moz-border-radius: 30px;
    border-radius: 30px;
    z-index: 1;
    opacity: 0;
    -webkit-animation: warn 3s ease-out;
    -moz-animation: warn 3s ease-out;
    animation: warn 3s ease-out;
    -webkit-animation-iteration-count: infinite;
    -moz-animation-iteration-count: infinite;
    animation-iteration-count: infinite;
}
</style>

</head>

<body>
<div class="container">
    <div class="dot"></div>
    <div class="pulse"></div>
</div>
</body>
</html>