Platforms that use Markdown-like syntax for formatting text but aren't actually compatible with Markdown. Gaze with me into the maw of madness:
_foo_ | *foo* | **foo** | |
|---|---|---|---|
| Markdown | italic | italic | bold |
| Google Chat | italic | bold | **unstyled** |
| Facebook Messenger | italic | bold | *bold & unstyled* |
| Slack | italic | bold | bold |
| Backloggd | underline | italic | bold |
These aren't even consistent enough that I can reliably remember "okay I should always use this one particular syntax for italics and that at least will always work". Nothing will always work. Life is suffering. At least Cohost is an island of standards-compliance in an ocean of nightmares.
my original philosophy behind this as someone pushing 40 was that my canonical ASCII renderings of bold, italic, and underline are *bold*, /italic/, and _underline_, which doesn't agree with any other site on the internet, so we might as well at least obey CommonMark
Or rather, it is following a completely different lineage that predates Markdown. Google Chat is copying, among other things, MSN Messenger1 and that is 5 years older than Markdown is. Markdown allowing _italic_ or *italic* feels like it is probably specifically appeasing chat apps using _italic_ even if it favours *italic*.
Underline and italic often traditionally had the same syntactic meaning. In Copy-editing you signal that something should be italicized by underlining it and apparently some publishers still use underline to mean italic pre-typesetting.
John Gruber chose wrong and now we all have to suffer for it2
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I'm assuming they took it from ICQ or AIM. And that one of those two got if from an earlier predecessor. I just know MSN did it because I was trying to work out where I picked up that convention just last week.
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There is a decent argument to make that in text where underscores may reasonably appear within words (e.g., variables, usernames), having underscore italicize causes too many problem.
