【ui_ol】
<!DOCTYPE html>
<html>
<head>
<style>
.dropdown ul {
display: none;
}
.dropdown:hover ul {
display: block;
}
</style>
</head>
<body>
<div class="dropdown">
Open
<ul>
<li>asdf</li>
<li>foo</li>
</ul>
</div>
</body>
</html>
