<!DOCTYPE html>
<html>
<head>
<title>index</title>
<script type="text/javascript" src="jquery-3.3.1.js"></script>
<style type="text/css">
</style>
</head>
<body>
<select>
<option value="1">Flowers</option>
<option value="2" selected="selected">Gardens</option>
<option value="3">Trees</option>
</select>
</body>
</html>
<script>
$(function(){
$("select option:selected").css({
"border" : "2px solid red",
"width" : "100px",
"background-color" : "red"
})
})
</script> 效果
正常是看不到的，只有将下拉框展示出来后，才会看到