How Can I Create a Series of Minimal App Windows That Only Consist of a Titlebar?

Read the help on the action.

Specify the attributes (like without a title) in the Gear menu, and specify the height and width of the window (several ways, but by setting the body attributes data-kmwidth and data-kmheight is easy).

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<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="utf-8" />
	<title>Custom HTML Prompt Example Testing</title>
	<style type="text/css"> 
body {
    max-height: 10px;
    margin: 0;
}
	</style> 
</head>
<body data-kmwidth="50" data-kmheight="25">
Hello
</body>
</html>