The most important thing is that I can run static analysis on a structured format. This is important even for my spec documents. I can write data fields and have static analysis analyze it. For example, to confirm database fields match across various spec documents, etc.. The static analysis is also why you use JSON/XML instead of HTML, since you can now have your own custom schema.
Also don't use YAML, as that's far more unreliable. (If you chop a YAML file in half, it's still valid)