Centered elements with Flex it is very easy to style, what about not use Flex?

<div class="wrapper">
  <div class="inner">Centered</div>
</div>

Table

It will make the inner element center horizontally and it works without setting a specific width.

.wrapper {
  display: table;
  margin: 0 auto;
}

Fixed width

Need to apply this CSS to the inner. You should set any width less than the containing wrapper will work. The margin: 0 auto is what does the actual centering.

.inner {
  max-width: 25%;
  margin: 0 auto;
}

Display: Inline-Block

display: inline-block makes the inner element into an inline element that can be centered with text-align: center

.wrapper {
  text-align: center;
}

.inner {
  display: inline-block;
}