flex-direction

Note that the values row and row-reverse are affected by the directionality of the flex container. If its dir attribute is ltr, row represents the horizontal axis oriented from the left to the right, and row-reverse from the right to the left; if the dir attribute is rtl, row represents the axis oriented from the right to the left, and row-reverse from the left to the right.

The following values are accepted:

row

The flex container's main-axis is defined to be the same as the text direction. The main-start and main-end points are the same as the content direction.

row-reverse

Behaves the same as row but the main-start and main-end points are opposite to the content direction.

column

The flex container's main-axis is the same as the block-axis. The main-start and main-end points are the same as the before and after points of the writing-mode.

column-reverse

Behaves the same as column but the main-start and main-end are opposite to the content direction.

Using the flex-direction property with values of row-reverse or column-reverse will create a disconnect between the visual presentation of content and DOM order. This will adversely affect users experiencing low vision navigating with the aid of assistive technology such as a screen reader. If the visual (CSS) order is important, then screen reader users will not have access to the correct reading order.

• Flexbox & the keyboard navigation disconnect — Tink
• Source Order Matters | Adrian Roselli
• MDN Understanding WCAG, Guideline 1.3 explanations
• Understanding Success Criterion 1.3.2 | W3C Understanding WCAG 2.0

flex-direction = 
  row             |
  row-reverse     |
  column          |
  column-reverse  

HTML

<h4>This is a Column-Reverse</h4>
<div id="col-rev" class="content">
  <div class="box red">A</div>
  <div class="box lightblue">B</div>
  <div class="box yellow">C</div>
</div>
<h4>This is a Row-Reverse</h4>
<div id="row-rev" class="content">
  <div class="box red">A</div>
  <div class="box lightblue">B</div>
  <div class="box yellow">C</div>
</div>

CSS

.content {
  width: 200px;
  height: 200px;
  border: 1px solid #c3c3c3;
  display: flex;
}

.box {
  width: 50px;
  height: 50px;
}

#col-rev {
  flex-direction: column-reverse;
}

#row-rev {
  flex-direction: row-reverse;
}

.red {
  background-color: red;
}

.lightblue {
  background-color: lightblue;
}

.yellow {
  background-color: yellow;
}

Result

• CSS Flexbox Guide: Basic Concepts of Flexbox
• CSS Flexbox Guide: Ordering flex items