How to display list items as columns preserving the left-to-right order?

Question

Is it possible in pure CSS to lay out list elements to arbitrary number of columns, preserving the left-to-right order, as on this example?

Answer 1

Yes, it should be theoretically possible.

• Since you want the flex items arranged in columns,

  #flex-container { flex-flow: column wrap; }
  

• But then the order of the elements would be preserved vertically (in columns). Since you want horizontally, they must be reordered:

  #flex-container > :nth-child(4n - 2) { order: 1; }
  #flex-container > :nth-child(4n - 1) { order: 2; }
  #flex-container > :nth-child(4n - 0) { order: 3; }
  

• And then we must force column breaks.

  According to 10. Fragmenting Flex Layout and CSS Fragmentation, line breaks can be forced with break-before:

  #flex-container > :nth-child(-n + 4) {
    page-break-before: always; /* CSS 2.1 syntax */
    break-before: always;  /* New syntax */
  }
  

  However, forced breaks in flexbox are not widely implemented yet. It works on Firefox, though.

#flex-container {
  display: flex;
  flex-flow: column wrap;
}
#flex-container > :nth-child(4n - 2) { order: 1; }
#flex-container > :nth-child(4n - 1) { order: 2; }
#flex-container > :nth-child(4n - 0) { order: 3; }
#flex-container > :nth-child(-n + 4) {
  page-break-before: always; /* Old syntax */
  break-before: always;  /* New syntax */
  border-top: 1px solid;
}

<div id="flex-container">
  <span>1</span>
  <span>2</span>
  <span>3</span>
  <span>4</span>
  <span>5</span>
  <span>6</span>
  <span>7</span>
  <span>8</span>
  <span>9</span>
  <span>10</span>
</div>