While working on the styles of my site, I decided to hide a background video for mobile devices (bandwidth reasons). The video was placed in a wrapper to ensure it always occupies the whole available space.
The problem was that later I could not get the video element to be displayed at all. I spent half an hour debugging in multiple browsers and still could not get the element to show up. I was on the edge of calling someone for help when I noticed this:
Lesson learned: Always pay attention to the most basic styles (often without any media query restriction) that are applied to parent elements and are hiding on the top of your stylesheet files.