Custom gradient in border is not working when stacked

I have this example where I'm trying to make borders with gradients in CSS: https://codesandbox.io/s/sparkling-pine-uvdcj?file=/src/styles.css

As you can see, I am able to show the border in the first card, the one that is not stacked into a container (just a simple div with a background).

When I put the same component into another element (as the container), I'm not able to show the border at all.

What is the problem?

Here the CSS and HTML:

body {
  background-color: gray;
}

.container {
  background-color: black;
  padding: 10px;
  margin: 10px;
}

.card {
  position: relative;
  margin-top: 32px;
  width: 300px;
  height: 180px;
  border-radius: 32px;
  background: linear-gradient(45deg, #1b1a1f, #1b1a1f 65%, rgba(77, 58, 40, 1));
  box-shadow: 0px 10px 10px rgba(0, 0, 0, 0.2);
}

.card:after {
  content: "";
  position: absolute;
  display: block;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  height: 182px;
  width: 302px;
  border-radius: inherit;
  margin: -1px;
  background: linear-gradient(40deg, #1b1a1f, #1b1a1f 60%, #ffb966 90%);
  z-index: -1;
}

<div class="card"></div>
<div class="container">
  <div class="card"></div>
</div>

Solution

That is happening because you have given the .card:after z-index of -1. Whereas the default z-index is auto or you can think of it as 0.

So, the container has the z-index of auto which is higher than the z-index of .card:after. That is why it is behind the black background.

You can do so by:

Adding the below styles to the .container.

.container {
  position: relative;
  z-index: -2;
}

Check it in action on Codepen.