custom css properties

Can you use the value of custom CSS properties? Seems not.

HTML

<div>
  <div class="one">1:</div>
  <div class="two">2: Text <span class="five">5 - more text</span></div>
  <input class="three">
  <textarea class="four">4: Lorem Ipsum</textarea>
</div>

CSS

#parent > DIV {
    content: '{"p1": 6, "p2": "a2"}';
    -x-p3: test;
}