ACF HTML5

Automatic mode supports these tags which are allowed by editor features. E.g. if you configure format plugin to create <aside>, <article>, <footer>, etc. then editor won't strip these tags. However, in a default configuration editor features do not create HTML5 tags, because most of them are related to page structure, not document contents. Also, we cannot assume that someone will use html5shiv on his site to make these tags work on older IEs. That's why default configuration does not use unsafe tags. But of course, feel free to reconfigure editor :).