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
Copied to your clipboard1<html>2 <head>3 <style>4 .highlight {5 color: red;6 }7 </style>8 </head>9 <body>10 <div class="highlight">Hello, world</div>11 </body>12</html>