head
Since UXP v2.0
Specifies various metadata about the UXP HTML document, including additional styles, scripts, etc.
See: https://developer.mozilla.org/en/docs/Web/HTML/Element/head
Example
<html>
<head>
<style>
.highlight {
color: red;
}
</style>
</head>
<body>
<div class="highlight">Hello, world</div>
</body>
</html>