How to make z-index work?

Question

css

https://jsfiddle.net/nq6Lekav/
Need .block to be under .wrap. However, .block must remain nested.
HTML

<div class="wrap">
  <div class="block"></div>
</div>

css

.wrap {
  position: relative;
  z-index: 1;
}
.block {
  position: fixed;
  z-index: 0;
}

Answer 1

You cannot put a nested element through the z-index under the parent. IT IS FORBIDDEN! Make it more correct!

Answer 2

Just like this)
https://jsfiddle.net/j0zLfLc2/

Answer 3

https://jsfiddle.net/nq6Lekav/2/ if I understand correctly