After upgrading my raspberry pi to use LibreELEC 10 or 11 I no longer got anything displayed on my TV when I started up the system. I found that the reason was because the HDMI output is disabled by default unless the TV is fully on (not just in standby). I remember playing around with some config.txt settings a long time ago to fix HDMI issues, but in the latest LibreELEC versions the method has changed.

For some reason all the Google results came back with outdated information, even though LibreELEC 10 has been around for a while now. But after playing around with all the hdmi options I could find in config.txt I finally found that those options don't work any more, and that the real fix is much simpler.

The new RPi builds use the same method as the Generic builds which is described here. You just need to ssh into your raspberry pi and run the getedid script with the create option. Make sure you do this while your TV is turned on but after it is done, you should be able to turn your raspberry pi on before waking your TV up without it causing problems.

The default root password for LibreELEC is libreelec, and you can get your raspberry pi IP address by going to the System Info option in the Kodi settings menu.

$ ssh root@192.168.1.13    # whatever your rpi IP address is
Warning: Permanently added '192.168.1.13' (ED25519) to the list of known hosts.
root@192.168.1.13's password: 
##############################################
#                 LibreELEC                  #
#            https://libreelec.tv            #
##############################################

LibreELEC (community): nightly-20211018-d3745e7 (RPi2.arm)
LibreELEC2:~ # getedid create
successfully installed edid override for HDMI-A-1

Now your raspberry pi should always be outputting to HDMI even if it doesn't find the TV at startup. If you change your TV you will need to SSH into the pi again and run getedid delete then getedid create with the new TV connected to update the EDID.