不知道哪儿错了，jQuery库是对的。
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>dom对象转化成jQuery对象</title>
<script type="text/javascript" src="D:\ChromeCoreDownloads\jquery-3.3.1.js"></script>
</head>
<body>
<div>波波鱼</div>
<div>香锅</div>
<div>年糕火锅</div>
<script type="text/javascript">
var div=document.getElementsByTagName('div');
var $div= $(div);
var $first= $div.first();
$first.css=('color','red');
</script>
</body>
</html>
