I don't think there is a default folder for the HTML Prompt. It appears from this example, that you need to set a <base href>
in the HTML:
Fancy Mac-Like HTML Prompts Library - Macro Library
<html>
<head>
<script>
document.write('<base href="' + window.KeyboardMaestro.GetVariable('htmlAssetsFolder') + '">');
</script>
<title>Dialog Box</title>
<script src="js/jquery.js"></script>
<script src="js/jquery-ui.js"></script>
<link rel="stylesheet" href="css/jqueryui.css">
<link rel="stylesheet" href="css/macdialog.css">
<script>