Keep the middle item centered when side items have different widths

If the left and right boxes would be exactly the same size, I get the desired effect. However when one of the two is a different size the centered box is not truly centered anymore. Is there anyone that can help me?

Here’s a method using flexbox to center the middle item, regardless of the width of siblings.

Key features:

• pure CSS
• no absolute positioning
• no JS/jQuery

Use nested flex containers and auto margins:

.container {
  display: flex;
}
.box {
  flex: 1;
  display: flex;
  justify-content: center;
}

.box:first-child > span { margin-right: auto; }

.box:last-child  > span { margin-left: auto;  }

/* non-essential */
.box {
  align-items: center;
  border: 1px solid #ccc;
  background-color: lightgreen;
  height: 40px;
}
p {
  text-align: center;
  margin: 5px 0 0 0;
}

<div class="container">
  <div class="box"><span>short text</span></div>
  <div class="box"><span>centered text</span></div>
  <div class="box"><span>loooooooooooooooong text</span></div>
</div>
<p>&#8593;<br>true center</p>

Here’s how it works:

• The top-level div (.container) is a flex container.
• Each child div (.box) is now a flex item.
• Each .box item is given flex: 1 in order to distribute container space equally (more details).
• Now the items are consuming all space in the row and are equal width.
• Make each item a (nested) flex container and add justify-content: center.
• Now each span element is a centered flex item.
• Use flex auto margins to shift the outer spans left and right.

You could also forgo justify-content and use auto margins exclusively.

But justify-content can work here because auto margins always have priority.

8.1. Aligning with auto
margins

Prior to alignment via justify-content and align-self, any
positive free space is distributed to auto margins in that dimension.