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.

True.

Perhaps //italic text// would be acceptable.

// is common in URIs.

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/.

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

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.