<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<meta http-equiv="X-UA-Compatible" content="ie=edge" />
<title>Document</title>
<style>
ul li:only-child {
color: red;
}
ul li:first-child:nth-last-child(2),
ul li:first-child:nth-last-child(2) ~ li {
color: green;
}
ul li:first-child:nth-last-child(3),
ul li:first-child:nth-last-child(3) ~ li {
color: blue;
}
</style>
</head>
<body>
<ul>
<li>1.1</li>
</ul>
<ul>
<li>2.1</li>
<li>2.2</li>
</ul>
<ul>
<li>3.1</li>
<li>3.2</li>
<li>3.3</li>
</ul>
</body>
</html>
refer