通过classu获取到一组元素，获取到的是一个类数组;
* 主语
* document 从整个文档中去获取class元素
* 父级 从父级去获取class元素
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title></title>
<script>
window.onload=function(){
var greens=document.getElementsByClassName("green");
console.dir(greens);
var color=document.getElementById('color');
var greens1=document.getElementsByClassName('green');
console.log(greens1);
}
</script>
</head>
<body>
<ul id="color">
<li>red</li>
<li class="green">green</li>
<li>blue</li>
<li>yellow</li>
<li>pink</li>
</ul>
<ul>
<li>red</li>
<li class="green">green</li>
<li>blue</li>
<li>yellow</li>
<li>pink</li>
</ul>
</body>
</html>
