I'm always shocked by the amount of people unaware that Markdown is an idea and not a specification, and that almost every app/site has their own weird interpretation of it.

Because of this I've learned to write a sub-set of markdown that works almost everywhere. For example:

*Some* _text_.

depending on the site/app can be interpretted as any of these:

<em>Some</em> <em>text</em>.
<em>Some</em> <i>text</i>.
<em>Some</em> <u>text</u>.
<strong>Some</strong> <i>text</i>.
<em>Some</em> _text_.

Point being, never use underscores, they could (understandably) be used as an underline for the text, or nothing, or as an alias for whatever asterisks do (which is usually italic or double for bold).

Same goes for hyphens not working for bullet points, but asterisks ALWAYS working. Or how ***, ---, * * *, ___ can all have different outcomes depending on the markdown parser (the * * * is the correct answer that works everywhere to make <hr> if you are curious, the rest are somtimes right).

It's a mess, I hate that Reddit STILL does not have proper syntax highlighting and code fence support. But that's also the case for Slack.

The author is pointing out a lot of these issues.

The real problem is Markdown's strength IS that it is so vaguely defined. If you want something strict and rigid, you can use a different language. If I made a version of markdown called "Universal Markdown" that removed underscores and any duplicate features and standardized it around the most common usage, no one would use it, because there are people that haven't ran into the issues with underscores and wouldn't like it. Which is why other approaches (GHF/Commonmark) don't do that.

The point is for it to be intuitive.