You're right, sadly it doesn't work with mplayer directly. I used to run
wget -O - | mplayer -
but now that also broke because of some seek issues.
It should be trivial though to change it to
wget -q -O /tmp/yt & (sleep 3; mplayer /tmp/yt)
I'll give you an example of my issue with youtube though: I get a rendering that looks like CSS is disabled and the message "This video is unavailable."
Now with new SSL providers giving out certificates that are good for only few weeks certificate pinning sadly got less useful for the general internet
One issue I have on youtube still is that clicking the "Stream" button is kind of difficult cause there's that defunct play button from youtube blocking most of it