Yes, in order for the z-index to work, you'll need to give the element a position: absolute or a position: relative property.
But... pay attention to parents!
You have to go up the nodes of the elements to check if at the level of the common parent the first descendants have a defined z-index.
All other descendants can never be in the foreground if at the base there is a lower definite z-index.
In this snippet example, div1-2-1 has a z-index of 1000 but is nevertheless under the div1-1-1 which has a z-index of 3.
This is because div1-1 has a z-index greater than div1-2.
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.div {
}
#div1 {
z-index: 1;
position: absolute;
width: 500px;
height: 300px;
border: 1px solid black;
}
#div1-1 {
z-index: 2;
position: absolute;
left: 230px;
width: 200px;
height: 200px;
top: 31px;
background-color: indianred;
}
#div1-1-1 {
z-index: 3;
position: absolute;
top: 50px;
width: 100px;
height: 100px;
background-color: burlywood;
}
#div1-2 {
z-index: 1;
position: absolute;
width: 200px;
height: 200px;
left: 80px;
top: 5px;
background-color: red;
}
#div1-2-1 {
z-index: 1000;
position: absolute;
left: 70px;
width: 120px;
height: 100px;
top: 10px;
color: red;
background-color: lightyellow;
}
.blink {
animation: blinker 1s linear infinite;
}
@keyframes blinker {
50% {
opacity: 0;
}
}
.rotate {
writing-mode: vertical-rl;
padding-left: 50px;
font-weight: bold;
font-size: 20px;
}<div class="div" id="div1">div1</br>z-index: 1
<div class="div" id="div1-1">div1-1</br>z-index: 2
<div class="div" id="div1-1-1">div1-1-1</br>z-index: 3</div>
</div>
<div class="div" id="div1-2">div1-2</br>z-index: 1</br><span class='rotate blink'><=</span>
<div class="div" id="div1-2-1"><span class='blink'>z-index: 1000!!</span></br>div1-2-1</br><span class='blink'> because =></br>(same</br> parent)</span></div>
</div>
</div>