It’s impossible without CSS. In fact, the <u> tag is simply adding text-decoration:underline to the text with the browser’s built-in CSS.
Here’s what you can do:
<html>
<head>
<!-- Other head stuff here, like title or meta -->
<style type="text/css">
u {
border-bottom: 1px dotted #000;
text-decoration: none;
}
</style>
</head>
<!-- Body, content here -->
</html>