[html] How to center horizontally div inside parent div

How do I center a div horizontally inside its parent div with CSS?

<div id='parent' style='width: 100%;'>
 <div id='child' style='width: 50px; height: 100px;'>Text</div>
</div>

<div id='child' style='width: 50px; height: 100px; margin:0 auto;'>Text</div>

Just out of interest, if you want to center two or more divs (so they're side by side in the center), then here's how to do it:

<div style="text-align:center;">
    <div style="border:1px solid #000; display:inline-block;">Div 1</div>
    <div style="border:1px solid red; display:inline-block;">Div 2</div>
</div>   

You can use the "auto" value for the left and right margins to let the browser distribute the available space equally at both sides of the inner div:

<div id='parent' style='width: 100%;'>
   <div id='child' style='width: 50px; height: 100px; margin-left: auto; margin-right: auto'>Text</div>
</div>

.parent-container {
  display: flex;
  justify-content: center;
  align-items: center;
}

.child-canvas {
  flex-shrink: 0;
}

<div id='parent' style='width: 100%;text-align:center;'>
 <div id='child' style='width:50px; height:100px;margin:0px auto;'>Text</div>
</div>