You need a semicolon after font-family: Arial, Helvetica, sans-serif. This will make your updated code the following:
<!DOCTYPE>
<html>
<head>
<title>DIV Font</title>
<style>
.my_text
{
font-family: Arial, Helvetica, sans-serif;
font-size: 40px;
font-weight: bold;
}
</style>
</head>
<body>
<div class="my_text">some text</div>
</body>
</html>