Aligning elements left and center with flexbox

EDIT: See Solo's answer below, it is the better solution.

The idea behind flexbox is to provide a framework for easily aligning elements with variable dimensions within a container. As such, it makes little sense to provide a layout where the width of one element is totally ignored. In essence, that is exactly what absolute positioning is for, as it takes the element out of the normal flow.

As far as I know, there is no nice way of doing this without using position: absolute;, so I would suggest using it... but If you REALLY don't want to, or can't use absolute positioning then I suppose you could use one of the following workarounds.

If you know the exact width of the "Left" div, then you could change justify-content to flex-start (left) and then align the "Center" div like this:

#center {
    position: relative;
    margin: auto;
    left: -{half width of left div}px;
}

If you do not know the width, then you could duplicate "Left" on the right side, use justify-content: space-between;, and hide the new right element: Just to be clear, this is really, really ugly... better to use absolute positioning than to duplicate content. :-)

#parent {
  align-items: center;
  border: 1px solid black;
  display: flex;
  justify-content: space-between;
  margin: 0 auto;
  width: 500px;
}
#right {
    opacity: 0;
}

<div id="parent">
  <span id="left">Left</span>
  <span id="center">Center</span>
  <span id="right">Left</span>
</div>

.parent {
  display: flex;
}

.left {
  flex: 1;
}

.parent::after {
  flex: 1;
  content: '';
}

<div class="parent">
  <div class="left">Left</div>
  <div>Center</div>
</div>

I have another solution. In my opinion, Adding an empty block to the center element is fine but code-wise it bit ugly.

Since this is 4 years old I figured I'd update this with a much easier CSS Grid solution.

#parent {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  border: 1px solid black;
  margin: 0 auto;
  width: 500px;
}
#center {
  text-align: center;
}

<div id="parent">
  <span id="left">Left</span>
  <span id="center">Center</span>
</div>

Add third empty element:

<div class="parent">
  <div class="left">Left</div>
  <div class="center">Center</div>
  <div class="right"></div>
</div>

And the following style:

.parent {
  display: flex;
}
.left, .right {
  flex: 1;
}

Only left and right are set to grow and thanks to the facts that...

• there are only two growing elements (doesn't matter if empty) and
• that both get same widths (they'll evenly distribute the available space)

...center element will always be perfectly centered.

This is much better than accepted answer in my opinion because you do not have to copy left content to right and hide it to get same width for both sides, it just magically happens (flexbox is magical).

In action:

.parent {
  display: flex;
}

.left,
.right {
  flex: 1;
}


/* Styles for demonstration */
.parent {
  padding: 5px;
  border: 2px solid #000;
}
.left,
.right {
  padding: 3px;
  border: 2px solid red;
}
.center {
  margin: 0 3px;
  padding: 3px;
  border: 2px solid blue;
}

<div class="parent">
  <div class="left">Left</div>
  <div class="center">Center</div>
  <div class="right"></div>
</div>