This attribute puts the information if the text was typed left to right (default for nodes) or right to left, being used to receive the typed value and know how it was typed.
Makes sense if used in conjunction with the style direction
and in languages such as Arabic.
Take the example:
<span>Texto para pesquisar:</span>
<input type="text" name="pesquisa" style="direction:RTL" dirname="pesquisaDirecao" />
If you submit the field, the query string will receive:
"?search=texto&research"
indicating that the search value was typed "rtl" that is, from right to left. If typed from left to right would post "search"
Here is a site explaining and where it is possible to test Submit: https://www.wufoo.com/html5/dirname-attribute/