<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>W3Cschool教程(w3cschool.cn)</title>
<style>
input:required
{
background-color: yellow;
}
</style>
</head>
<body>
<h3>A demonstration of the :required selector.</h3>
<p>An optional input element:<br><input></p>
<p>A required input element:<br><input required></p>
<p> :required选择器选择表单元素有“需要”属性.</p>
</body>
</html>
