使用：first-child伪类为元素添加特殊样式，该元素是某个其他元素的第一个子元素。
您可以尝试运行以下代码来了解：first-child伪类的用法-
<html>
<head>
<style>
div > p:first-child
{
text-indent: 25px;
}
</style>
</head>
<body>
<div>
<p>First paragraph in div. This paragraph will be indented</p>
<p>Second paragraph in div. This paragraph will not be indented</p>
</div>
<p>But it will not match the paragraph in this HTML:</p>
<div>
<h3>Heading</h3>
<p>The first paragraph inside the div. This paragraph will not be effected.</p>
</div>
</body>
</html>