Where is there an explanation of the HDMI interface and its parameters?

I just received an email from a friend asking:
Do you know anyone who's an expert in the HDMI interface and any parameters that might affect it? (Or a site with a blindingly clear explanation of the whole technology.) I'm trying to track down a strange problem that appears to be down at the device driver level. It's probably blindingly obvious to one skilled in the art, but I'm not.
I could say that I'm not an expert either (but this would be code for "I don't have a clue"). Does anyone know a suitable online resource?

For the simplest starting point, I always go to Wikipedia: HDMI Protocol
The thing about high level protocols like HDMI, is that they take something very, very complex and make it appear simple.  On the surface, you just plug your source into a display and it just works, but what goes on "behind the scenes" is very complicated.  The source and device must first communicate to determine the highest (best) video resolution that both devices can support. Then they look to see which audio resources are present,  Sometimes there's also an Ethernet connection in there too. Then there's "Digital Rights Management" that's also part of the protocol.As far as troubleshooting a problem that's down at the device driver level- all I would know how to do is to try a different device and see what effect that has on the problem. -Rick    

Analog devices have some good guides as they manufacture HDMI devices. From their website I found this:https://www.eetimes.com/document.asp?doc_id=1273813
HDMI is based on DVI and uses the same Transmission Minimised Differential Signalling (TMDS) which requires some careful attention with signal reflections and cable lengths. It is possible to obtain the DVI spec but the HDMI spec you have to pay for and you may need an NDA.
HDMI has Digital Rights Management in the form of the High-bandwidth Digital Content Protection (HDCP) function. If the content being streamed requires HDCP, you must have compatible display driver and receiver (monitor). HDCP uses cryptographic keys, obtainable once you pay licence fees and sign the HDMI NDA.You also need to ensure you read and process the data from the EDID ROM on the display and scale your output video, if required to do so.

The HDMI Forum (hdmi.org) drive the technical standards and adopters.  I'm no expert, but I have played around with HDMI and before that DVI interfaces to Silicon Image (now owned by Lattice) chips at the driver level.  It was a long time ago but if your friend posts an explanation of the problem they are having then someone on the forum may have a light-bulb moment.