I'm not claiming you claimed it was a simple specification :-)
I just find it interesting. This would indicate to me that there are 500 "features" in the language. I thought mark-down languages just provided a few shortcuts for producing the most commonly needed HTML features and then provide a fallback to HTML. So if you cannot do it in the markdown language, use HTML instead.
I can't really be bothered to take a look at the tests, but I strongly doubt there are actually 500 features. A large part of those tests are probably trying combinations of features. E.g. suppose markdown only had tables as a feature, and nothing else. That feature alone deserves several several tests (for tables of various sizes, edge cases such as having only the header, having rows with an incorrect number of columns, etc.).
But let's assume we can get away with just a single test for tables. And then we introduce the features "section headers" and "bold" and "underline". All these features can interact (e.g. underlined bold section headers), so we want to test combinations of all those features, and have a nice combinatorial explosion.
Why would I care if one is merely “formatting” or not? If I have to run a tool either way, I would prefer one that accepts a user-friendly input language and decouples content from presentation.
Because transforming an .md file into an .html file is a lot more invasive (though taken for granted here I think) than just writing the .html file. It's a build step where there wasn't one before.
There are several for HTML different versions and it is standardized that you can omit some closing tags and some tags altogether.
The benefit of writing in a standardized language is that later you or anybody can run tools against your sources that check for conformity.
So that is why I prefer HTML. But I would like to hear your opinion on what is the best mark-down dialect currently?