How to align absolutely positioned element to center?

I am trying to stack two canvas together and make it a double layers canvas.

I've saw an example here:

<div style="position: relative;">
 <canvas id="layer1" width="100" height="100" 
   style="position: absolute; left: 0; top: 0; z-index: 0;"></canvas>
 <canvas id="layer2" width="100" height="100" 
   style="position: absolute; left: 0; top: 0; z-index: 1;"></canvas>
</div>

But i would like to set both of the canvas align at the center of the screen. If i set the value of left as a constant, while I change the orientation of the screen (as I'm doing aps on iPad) the canvas won't remain at the middle of the screen like how it act in

<div align="center">

Can anyone help, please?

If you set both left and right to zero, and left and right margins to auto you can center an absolutely positioned element.

position:absolute;
left:0;
right:0;
margin-left:auto;
margin-right:auto;

If you want to center align an element without knowing it's width and height do:

position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);

Example:

*{
  margin:0;
  padding:0;
}
section{
  background:red;
  height: 100vh;
  width: 100vw;
}
div{  
  width: 80vw;
  height: 80vh;
  background: white;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

<section>
  <div>
    <h1>Popup</h1>
  </div>
</section>

try this method, working fine for me

position: absolute;
left: 50%;
transform: translateX(-50%);