Damping resistors on HDMI Input

In a perfect world, 0 Ohm resistors wouldn't do anything.

In reality, they only have parasitic effects — namely, primarily a parasitic impedance (says Vishay).

Now, there's two different possible reasons the people at MStar recommend using 0R in your signal lines:

1. The person you've talked to isn't that involved in the design of PCBs or the IC and was under the misconception that where, for example, their demo board had 0R resistors for flexibility, you should have such, too, or
2. You actually need these parasitic effects.

Option 2. is actually the more interesting one. It clearly indicates that MStar can't or didn't guarantee a good impedance matching for their differential endpoints – otherwise, without any doubt, the "plain" differential transmission line with the characteristic impedance their datasheet claims would be the optimal connection.

So, please do two things: Ask back; be polite. Engineers and support people are human, too! Also, report back. Having a non-perfectly matched real impedance isn't terrible, but it of course calls for knowledge of that fact if you want to build a reliable device.

It's perfectly possible that MStar has experience we don't have – for example, it might be that HDMI sources typically don't adhere to spec themselves, or having a bit of an inductance to compensate for capacity effects in HDMI cabling etc.

EDIT: you received a reply from MStar's distributore:

Please still need you to put back the 0R. It is not just for EMI and it also for impedance test so we can fine tune it when signal is not so good. This recommend from MStar.

In other words, they tell you through the flower "please don't trust your design skills and/or the specs of our components overly much; it's usual to exchange the 0R for something that compensates for factual mismatch later on".

Which frankly, isn't that bad of advice (considering 0R isn't really expensive, and placing a couple of SMD resistors in an automated process isn't either), but also means that

1. specs are to be handled with a grain of distrust and
2. you can't use resistor networks (like the ones I infer from your schematic), but should use individual resistors so that you can correct the individual differential pairs if necessary.