Since WordPress 3.6, it’s been possible for theme developers to optionally add HTML5 support for search forms, comment forms, and comment lists. With WordPress 3.9, that optional support will be extended to galleries.
The new default gallery markup will use
<figcaption> for those themes that enable it.
This is a good thing. And Konstantin Obenland has put together a quick snippet to show theme authors how to enable support for this feature, as well as create both forward and backward compatibility, as they’ve done with the Underscores theme.