<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<p class="cla" >我要学习0</p>
<div id="id" class="cla" name="na">我要学习1
<p class="cla" >我要学习2</p>
<div class="cla" name="na">我要学习3</div>
<p class="cla">我要学习4</p>
<div class="cla" name="na">我要学习5</div>
</div>
<script type="text/javascript">
var z=document.getElementById("id").getElementsByTagName("p");
t=z.length
for (var i=0;i<t;i++){
z[i].style.color="red";
}
</script>
</body>
</html>
返回值：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<p class="cla" >我要学习0</p>
<div id="id" class="cla1" name="na">我要学习1
<p class="cla" >我要学习2</p>
<div class="cla1" name="na">我要学习3</div>
<p class="cla">我要学习4</p>
<div class="cla1" name="na">我要学习5</div>
</div>
<script type="text/javascript">
var z=document.getElementById("id").getElementsByClassName("cla1");
t=z.length
for (var i=0;i<t;i++){
z[i].style.color="red";
}
</script>
</body>
</html>
正确的做法只有上面两种，并且id选择器必须在前。
以下四种方法均为错误：
var z=document.getElementById("id").getElementsByName("na");
var z=document.getElementById("id").getElementsByClassName("id");
var z=document.getElementsByClassName("cla").getElementsById("id");
var z=document.getElementsByClassName("cla").getElementsByTagName("na");