style.css
index.html
/* By default, paragraphs are red. */
p { color: red; }
/* If the paragraph has an anchor in it, it becomes black. */
p:has(a) { color: black; }
<!DOCTYPE html>
<html>
<head>
<title>:has()</title>
<link href="style.css" rel="stylesheet">
</head>
<body>
<p>This is a paragraph without a link in it.</p>
<p>This is a paragraph with <a href="#">a link</a> in it.</p>
</body>
</html>