Way back in the day when I was on IRC, I usually read them as,

  *bold*
  /italics/
  _underline_

… which would have made sense, if you stop to think about the characters. (But of course humans are wibbly-wobbly in what they'll choose, so context is context.) Alas, bold is now different in everything, it seems, and I can never remember which is which, aside from Discord which gives visual feedback.

And even HTML was not safe because <b> and <i> were too easy so now we have <strong> and <em> because somehow those are more meaningful.

The worst part of the <i> to <em> migration is now <em> often gets used for book titles, which is completely wrong. Book titles aren’t emphasized, they’re italicized.

If someone hits control+I, or an italics button, it should use <i> tags, not <em>.

I think one of Markdown's biggest mistake was making `*` and `_` interchangeable. It should have been `*bold*` and `_italics_` from the beginning.

I agree with you on that.

What about * for bold, / for italic and _ for underline?

/ is too commonly used, such as when making a list of replacements/alternatives/synonyms. It works fine when it's just the text as written, but if it was parsing and turned that list to replacementsalternativessynonyms, it would only be getting in the way.

You shouldn't parse '/' in foo/bar/baz or in foo / bar / baz.

If people want to make foo or foo bar italic, they write /foo/ and /foo bar/.

True.

Perhaps //italic text// would be acceptable.

// is common in URIs.

HN uses *italics*.

I, myself, prefer /italics/, _underline_ and _bold_ in text, if there is no formatting. I feel that these 2 are the least arbitrary, they almost work without agreeing on their meaning beforehand.

I have no regrets on this one. I thought people would use both, and people do use both.