html

<?xml version="1.0"?>

js

let parser = new DOMParser();
const doc = parser.parseFromString(
  '<?xml version="1.0"?><test/>',
  "application/xml",
);
const pi = doc.createProcessingInstruction(
  "xml-stylesheet",
  'href="mycss.css" type="text/css"',
);
doc.insertBefore(pi, doc.firstChild);

const output = document.querySelector("output");
output.textContent = `This processing instruction's target is: ${doc.firstChild.target}`;

html

<?xml version="1.0"?>
<pre></pre>

js

const node = document.querySelector("pre").previousSibling.previousSibling;
const result = `Node with the processing instruction: ${node.nodeName}: ${node.nodeValue}\n`;
document.querySelector("pre").textContent = result;