<!DOCTYPE html>
<html>
<head>
<style type='text/css'>
ul>li:hover {
font-weight: bold;
}
ul>li:hover li {
font-weight: normal;
}
</style>
</head>
<body>
<ul>
<li>A
<ul>
<li>subection</li>
</ul>
</li>
<li>B
<ul>
<li>subection</li>
</ul>
</li>
<li>C
<ul>
<li>subection</li>
</ul>
</li>
</ul>
</body>
</html>