<acronym> element is obsolete.

Created originally on Bitbucket by Russ (Russ W)

When running this:

#!rst

 an acronym (:ac:`reST`)

and get this HTML output:

#!html
an acronym <acronym>reST</acronym>

And then put the HTML output through the HTML5 validator this error pops up:

Error Line 194, Column 29: The acronym element is obsolete. Use the abbr element instead.

                    <acronym>reST</acronym>

Solution: replace the transform to use the abbr element.