The problem:
At some videos at YouTube you will receive the message that the video can't be played.
If you click on the mostly asked questions about HTML5 you will probably see that your system does not support H.264.

H.264 is a video codec and you have to install it at Fedora manualy - if it is okay for you to use non-free rep.
so you can fix it with these steps:

1. Add the non-free rep. of Fedora:
sudo dnf install$(rpm -E %fedora).noarch.rpm$(rpm -E %fedora).noarch.rpm

If you will be asked to accept the security keys hit yes.

2. Update your reps.:
sudo dnf update

3. Install the libs for mpeg (including H.264):
sudo dnf install ffmpeg-libs

Restart your browser and now you can play videos with H.264 codec.