I can play some mkv videos but some others can't play. Why? I don't want to install
ubuntu-restricted-extras cause it brings Adobe Flash that has not been supported again and it is the old version. And it is not easy to uninstall.
You just have to install the gstreamer ugly plugins that includes the h264 codecs:
So, just run:
and it's done. You don't need to install anything else, nor use the ubuntu-restricted-extras package.