How to keep z-index order on multiple elements transform animation?

Question

I just want to keep z-index order as it is before presing alone-box toggle animation button. jsfiddle

function toggle_class() {if(document.getElementById('alone-boxes-wrapper').className == 'animated') document.getElementById('alone-boxes-wrapper').classList.remove('animated'); else document.getElementById('alone-boxes-wrapper').classList.add('animated');}

div.alone-box {
  position:absolute;
  z-index: 1;
  height: 100px;
  width:100px;
  background-color: #F4F1DE;
  opacity:0.9;
  border:1px dashed #000;
}

div.absolute-wrapper {
  position:absolute;
  top:20px;
}


div.wrapper {
  position:relative;
  height: 100px;
  width:100px;
  left: 50px;
}

div.wrapper .box-1 {
  position:absolute;
  z-index:0;
}

div.wrapper .box-2 {
  position:absolute;
  top: 20px;
  z-index:2;
}


@keyframes animation {
    from { transform: translateX(1rem); }
    to { transform: translateX(-1rem); }
}


div.wrapper.animated .box-1 {
  animation-name: animation;
  animation-duration: 5s;
  animation-iteration-count: infinite;
  animation-direction: alternate;
  animation-timing-function: linear;
}

#alone-boxes-wrapper.animated {
  animation-name: animation;
  animation-duration: 5s;
  animation-iteration-count: infinite;
  animation-direction: alternate;
  animation-timing-function: linear;
}

<div id="alone-boxes-wrapper">
  <div class="alone-box"></div> <!-- z-index: 1 -->
</div>


<div class="absolute-wrapper">

  <div class="wrapper animated">
    <div class="box-1">1111111111</div> <!-- z-index: 0 -->
    <div class="box-2">222222222</div> <!-- z-index: 2 -->
  </div>

</div>



<button type="button" style="position:absolute;top:120px" onClick="toggle_class()">alone-box toggle animation</button>

Answer 1

transform acts like position:relative (if no position is set) , so z-index default value comes to be 0 and the child box follows this new stacking context.

The child boxe is indeed z-index:1 but within the parent at z-index:0. Since it is drawn first, next box drawn on same level will be drawn over it.

an example with static, relative , transform and z-index : http://codepen.io/anon/pen/AXkzOB

you need to set the z-index on the absolute boxe within the assigned class #alone-boxes-wrapper.animated

function toggle_class() {if(document.getElementById('alone-boxes-wrapper').className == 'animated') document.getElementById('alone-boxes-wrapper').classList.remove('animated'); else document.getElementById('alone-boxes-wrapper').classList.add('animated');}

div.alone-box {
  position:absolute;
  z-index: 1;
  height: 100px;
  width:100px;
  background-color: #F4F1DE;
  opacity:0.9;
  border:1px dashed #000;
}

div.absolute-wrapper {
  position:absolute;
  top:20px;
}


div.wrapper {
  position:relative;
  height: 100px;
  width:100px;
  left: 50px;
}

div.wrapper .box-1 {
  position:absolute;
  z-index:0;
}

div.wrapper .box-2 {
  position:absolute;
  top: 20px;
  z-index:2;
}


@keyframes animation {
    from { transform: translateX(1rem); }
    to { transform: translateX(-1rem); }
}


div.wrapper.animated .box-1 {
  animation-name: animation;
  animation-duration: 5s;
  animation-iteration-count: infinite;
  animation-direction: alternate;
  animation-timing-function: linear;
}

#alone-boxes-wrapper.animated {
  z-index: 1;/* here */
  animation-name: animation;
  animation-duration: 5s;  
  animation-iteration-count: infinite;
  animation-direction: alternate;
  animation-timing-function: linear;
}

<div id="alone-boxes-wrapper">
  <div class="alone-box"></div> <!-- z-index: 1 -->
</div>


<div class="absolute-wrapper">

  <div class="wrapper animated">
    <div class="box-1">1111111111</div> <!-- z-index: 0 -->
    <div class="box-2">222222222</div> <!-- z-index: 2 -->
  </div>

</div>



<button type="button" style="position:absolute;top:120px" onClick="toggle_class()">alone-box toggle animation</button>

or once for all in the id #alone-boxes-wrapper

function toggle_class() {if(document.getElementById('alone-boxes-wrapper').className == 'animated') document.getElementById('alone-boxes-wrapper').classList.remove('animated'); else document.getElementById('alone-boxes-wrapper').classList.add('animated');}

div.alone-box {
  position:absolute;
  z-index: 1;
  height: 100px;
  width:100px;
  background-color: #F4F1DE;
  opacity:0.9;
  border:1px dashed #000;
}

div.absolute-wrapper {
  position:absolute;
  top:20px;
}


div.wrapper {
  position:relative;
  height: 100px;
  width:100px;
  left: 50px;
}

div.wrapper .box-1 {
  position:absolute;
  z-index:0;
}

div.wrapper .box-2 {
  position:absolute;
  top: 20px;
  z-index:2;
}


@keyframes animation {
    from { transform: translateX(1rem); }
    to { transform: translateX(-1rem); }
}


div.wrapper.animated .box-1 {
  animation-name: animation;
  animation-duration: 5s;
  animation-iteration-count: infinite;
  animation-direction: alternate;
  animation-timing-function: linear;
}

#alone-boxes-wrapper.animated {
  animation-name: animation;
  animation-duration: 5s;
  animation-iteration-count: infinite;
  animation-direction: alternate;
  animation-timing-function: linear;
}
#alone-boxes-wrapper {
      z-index: 1;/* here */
  }

<div id="alone-boxes-wrapper">
  <div class="alone-box"></div> <!-- z-index: 1 -->
</div>


<div class="absolute-wrapper">

  <div class="wrapper animated">
    <div class="box-1">1111111111</div> <!-- z-index: 0 -->
    <div class="box-2">222222222</div> <!-- z-index: 2 -->
  </div>

</div>



<button type="button" style="position:absolute;top:120px" onClick="toggle_class()">alone-box toggle animation</button>

more info here : http://webdesign.tutsplus.com/articles/what-you-may-not-know-about-the-z-index-property--webdesign-16892 (shared from @seahorsepip )