z-index not working on pseudo-element

Question

I have encountered a weird issue, while trying to implement a custom background on website I'm working on.

I've written a proof of concept code piece on codepen and it works perfectly there, but when I try to implement it on website it does not.

body {
  background: black;
}

.background-test {
  background: white;
  width: 20%;
  margin: 2% 50%;
  height: 250px;
  padding: 1%;
  position: relative;
}

.background-test:before {
  width: 100%;
  height: 100%;
  content: "";
  background: red;
  position: absolute;
  z-index: -1;
  bottom: -3%;
  left: -2%;
  -webkit-clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
  clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
}

<div>
  <div class="background-test">Lorem Ipsum</div>
  <div></div>
</div>

https://codepen.io/Hassansky/pen/eEaQxG/

You can see that the :after pseudo element is positioned correctly - in theory. When I try to implement this into the website I'm working on, it just doesn't show. Chrome Dev tools shows it's there, just not displaying it. It does show up when I disable the z-index on dev tools, but then it stacks on top of the parent, as it should.

I tried to look for stacking issues, but I'm at wits end and cannot find any reasonable explanation for this.

This is a Wordpress website loaded with theme, but this should not present an issue with z-index stacking, especially when I cannot find any rule regarding z-indexes there.

Page url: http://polkrys.pl/cukiernia/podklady-cukiernicze-okragle-biale/

I've marked which elements should have pseudo-elements on them.

I'm adding SASS code that relates to elements in question:

.polkrys_shadow {
        position: relative;
        &:before {
            width: 100%;
            height: 100%;
            content: "";
            background: red;
            position: absolute;
            z-index: -1;
            bottom: -3%;
            left: -2%;
            -webkit-clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
            clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
        }

The margins and paddings are defined within wordpress visual composer. I suggest inspecting mentioned elements with Dev Tools - you'll see it should work - but it doesn't.

Answer 1

I found this solution:

el {
  transform-style: preserve-3d;
}

el:before {
  transform: translateZ(-1px);
}

Answer 2

The parent element of the :pseudo-element requires a z-index declared as well in order for the z-index of the :pseudo-element to function as intended. But doing so will stack the :pseudo-element over the parent element, concealing the background and nested content.

To rectify this, nested content should have a higher z-index declared. And an additional :pseudo-element (:after) should be declared to overlay the initial :pseudo-element (:before) with the background fill applied (e.g: "white"), to conceal the initial pseudo-element only allowing the overflow to reveal.

.logistic-bg .polkrys_shadow:after { /* Additional pseudo-element added */
   content: ""; 
   background: white; 
   position: absolute; 
   left: 0; 
   right: 0; 
   top: 0; 
   bottom: 0; 
}

.logistic-bg .polkrys_shadow { /* Adjustments made on parent element */
   z-index: 9;
}

.logistic-bg div:first-child { /* Adjustments made on nested element */
   position: relative; /* required for z-index to apply */
   z-index: 99;
}

Answer 3

<div>
  <div class="background-test">Lorem Ipsum <div style="z-index:100;color:blue;background-color:yellow;">vgghuu</div>
</div></div>

body {
  background: black;
}

.background-test {
  background: white;
  width: 20%;
  margin: 2% 50%;
  height: 250px;
  padding: 1%;
  position: relative;
  z-index: 1;
}

.background-test:before {
  width: 100%;
  height: 100%;
  content: "";
  background: red;
  position: absolute;
  z-index: -1;
  bottom: -3%;
  left: -2%;
  -webkit-clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
  clip-path: polygon(2% 4%, 100% 0, 99% 97%, 0 100%);
}