Flexbox and <fieldset>
don't mix well: http://jsfiddle.net/w9k5y/9/ So you better wrap flex-box instructions with a <div>
for now.
But for this case you don't even have to use flexbox. text-align: center;
brings you the same result: http://jsfiddle.net/w9k5y/1/ It's the same idea as Romain had, but you don't necessarily need the extra <div>
this time.