The VIA chipsets only support external HDMI, which could be connected through external HDMI transmitter, i.e. AD9389/AD9889.

To light HDMI, you need to confirm the HDMI is configured to be the active display device. The default value of HDMIPort depends on which VIA chipset you use. And if the HDMI could not be enabled by the default setting, you can use the following option to set the digital port.

Option "HDMIPort" " DFP_HIGH" (or "DFP_LOW", "DVP0", "DVP1")

DVP represents "Digital Visual Port".
DFP represents "Digital Flat Panel Port". They are all physical interfaces.

For example,

Section "Device"
       Driver   "via"
       VendorName  "VIA Tech"
       BoardName   "via"
       Identifier  "Device0"
       Option "ActiveDevice" "HDMI"
       Option "HDMIPort"       "DVP1"
       Screen 0 
EndSection