Every H700 device has a mini HDMI port on the top of it. Due to how muOS works, a check is done at boot time to check if an HDMI cable is plugged in. This means hot-plugging the HDMI will not work; to be safe, you should insert the HDMI cable while the device is completely off.
Resolutions
The devices can output 480i, 576i, 480p, 576p, and 720p(50hz/60hz). This can be set in General Settings > HDMI Output. If you pick a resolution your monitor does not support, it will not display anything, and it will look like the screen is frozen. If this happens, unplug the HDMI and do a safe reboot (L2 + R2 + Start).
Themes
If the theme supports it, the theme engine will use the HDMI resolution for the theme to display. Even if the theme does not support the current resolution, the theme engine will try its best to fit the current theme. On the themes site, you can find themes that support HDMI with the HDMI checkbox at the top.
Charging and Peripherals
Except for the RG35XXH, it is not possible to charge and use a plugged in device at the same time. (For the RG35XXH, 1 port can be used for both data and charging, while the other can be used for data.) A common way around this is through use of Bluetooth to connect a controller wirelessly.
Audio
You can choose to output audio Externally through HDMI or Internally under General Settings > HDMI Output. If using Bluetooth, you can also use the app to choose a Bluetooth audio source.
RG28XX Devices
Due to time constraints and custom code in the frontend application for the RG28XX, it is currently disabled by default.
You can enable it by going to the Simple Terminal application and typing in printf 1 >/run/muos/device/board/hdmi then exit to quit the application. Restarting the device you should see HDMI options in General Settings.
Weird behavior may occur with the RG28XX.