I’ve learned that you can reverse the sequence of numbers in an HTML ordered list (<ol>) with the HTML5 attribute, reversed. As in:

<ol reversed>
    <li>French hens</li>
    <li>Partridge in a pear tree</li>

Which gives:

  1. French hens
  2. Turtledoves
  3. Partridge in a pear tree

There is also the attribute start to specify the start value, useful to resume the numbering of a list after it is interrupted by another block element.