Videos don't display

Hi all,

I have the following issue. Videos inserted into my page content do not display as videos, instead the url (or shortcode) is shown as plain text.

This happens when I simply copy the URL into a page editor, when I use shortcodes in the page editor, and also when using shortcodes as part of a PHP page template (using the do_shortcode statement).

I’m not using a pre-made theme. Instead I’ve just created my own *empty* theme, and then filled it with my own style.css and page templates (as well as a nearly empty functions.php). So in this case, the site essentially IS the theme. Nothing of the things I’ve added should have anything to do with video playing.

Unlike in my own theme, however, all videos display (and play) fine in a standard theme like Twenty Fifteen. This leads me to suspect that video playing doesn’t simply work as an “out of the box” WordPress feature, but that instead something has to be done within the used theme (in this case, my own theme) in order to ‘enable’ video playing.

Anyone have an idea as to why my video’s won’t play in my own theme? Was I naive in thinking that video playing is an “out of the box” WordPress feature that should work even if you start with an empty theme?

Thanks.
Hagelslag.