<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style type="text/css">
.cla1{
background-color: blue;
color: red;
}
span.red{
color: green;
}
#sec{
color: black;
text-align: center;
}
</style>
</head>
<body>
<!-- class不唯一 -->
<div class="cla1">
<h1>this is div_h1</h1>
<p>text text</p>
</div>
<h1>My <span class="red">Important</span> Heading
</h1>
<!-- id唯一 -->
<h1 id="sec">SecondH1</h1>
</body>
</html>