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":nth-child(n)" is a selector in CSS. Its function is to match the Nth child element belonging to its parent element, regardless of the type of element; among them, the parameter "n" can be Numbers, keywords or formulas, such as ":nth-child(3)", ":nth-child(even)".
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In CSS, a selector is a pattern used to select elements that need to be styled. nth-child(n) is a selector in css3.
:nth-child(n) selector is used to match the child element whose parent element index is n, that is, to select the Nth child element belonging to its parent element, regardless of the type of element.
Note: n can be a number, keyword (Odd and even) or formula.
Example 1: When n is a number
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>:nth-child(n)选择器</title>
<style>
/*
:nth-child(n)选择器用于匹配父元素索引为n的子元素，即选取属于其父元素的第N个子元素，不论元素的类型。
n可以是数字、关键词（Odd 和 even）或公式。
*/
/* p:nth-child(2) {
background: pink;
} */
</style>
</head>
<body>
<p>第一个段落。</p>
<p>第二个段落。</p>
<p>第三个段落。</p>
<p>第四个段落。</p>
</body>
</html>Rendering:
Example 2: When n is a keyword (Odd and even )
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>:nth-child(n)选择器</title>
<style>
p:nth-child(odd) {
background: pink;
}
p:nth-child(even) {
background: paleturquoise;
}
</style>
</head>
<body>
<p>第一个段落。</p>
<p>第二个段落。</p>
<p>第三个段落。</p>
<p>第四个段落。</p>
<p>第五个段落。</p>
<p>第六个段落。</p>
<p>第七个段落。</p>
<p>第八个段落。</p>
<p>第九个段落。</p>
<p>第十个段落。</p>
</body>
</html>Rendering:
Example 3: When n is the formula
 "161174403722093What is the function of :nth-child(n)")
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>:nth-child(n)选择器</title>
<style>
/*
使用公式 (an + b)。描述：表示周期的长度，n 是计数器（从 0 开始），b 是偏移值。
在这里，我们指定了下标是 3 的倍数的所有 p 元素的背景色：
*/
p:nth-child(3n+0)
{
background:pink;
}
</style>
</head>
<body>
<p>第一个段落。</p>
<p>第二个段落。</p>
<p>第三个段落。</p>
<p>第四个段落。</p>
<p>第五个段落。</p>
<p>第六个段落。</p>
<p>第七个段落。</p>
<p>第八个段落。</p>
<p>第九个段落。</p>
<p>第十个段落。</p>
</body>
</html>Rendering:
Use formula (an b). Description: Indicates the length of the period, n is the counter (starting from 0), and b is the offset value.
Here, we specify the background color of all p elements whose subscripts are multiples of 3:
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