If you are like me using a TFT that is connected via SPI (e. g. PiTFT 2.8" 320x240) driven by FBTFT in combination with fbcp to utilise hardware accelerated video decoding (using omxplayer) like it is descriped here. You should add the following into the /boot/config.txt to force the output to HDMI and set the resolution to 320x240:
hdmi_force_hotplug=1
hdmi_cvt=320 240 60 1 0 0 0
hdmi_group=2
hdmi_mode=87
