This results in "Test 123" in a JSON-supporting browser, but throws an error in Prince:
prince: json.html:2: error: TypeError: value is not an object

<p>Test <span id="test"></span></p>

<script>
  var x = JSON.parse('{"y":123}');
  document.getElementById('test').textContent = x.y;
</script>

Good news, this is supported in Prince latest builds.

Awesome, thanks.