This should be marked with [2016]. Personally, I've switched to ASCIIDoc (after looking at reStructured Text) because Markdown has too many variants and they don't consistently extend the markup the same way (a document that looks good in visual code fails to render in Github and vice-versa).

If you don't mind me asking, what was the closest you came to finding a 'standard' Markdown? I use a subset frequently enough that it tends not to be an issue but there are times when, for example, embedding latex formatted math would be useful in a uniform manner.

Since we use Gitlab for private repositories and Github for public ones, the main irritation I find is that even those two aren't completely compatible. From a standards perspective, the "official" seems to be supported everywhere but when you start adding in "extensions/variants" all bets are off. You might be able to write most blog posts with the official version but I like TODO lists, tables, etc (without resorting to inline HTML).

I was surprised to see that this came from rcn.com, they used to be my ISP. Can someone give context here? Who is David Moon? What does he have to do with RCN?

Moon used to be famous among hackers: https://en.wikipedia.org/wiki/David_A._Moon ("One day a student came to Moon and said: “I understand how to make a better garbage collector. We must keep a reference count of the pointers to each cons.” Moon patiently told the student the following story: “One day a student came to Moon and said: ‘I understand how to make a better garbage collector...")

Oh it looks like you can set up a user page under their users subdomain.

This was precisely my experience

It's oddly specific that sequence diagrams can be expressed in the language, but not images.

You can embed an image by embedding html (via the html 'directive') I suppose.

Embedding images inside markdown is really where things break down from the natual very text orientated and direct workflow of writing markdown. All of a sudden you have to fiddle with urls or files, somehow put your image somewhere, and then making sure the markdown link you made works correctly.

This particular markdown variation looks like it is tailored for technical documentation and there the sequence diagrams make a lot of sense.

Well, directives are (relatively) easy to add, at least in the Common Lisp version that I read through. So adding a directive to display images or other things would be a straightforward addition to someone using the format.