Where the hell did you get HTML from? /s is a tag literally made to denote that a given text is sarcastic. It’s one of the few good things to come out of Twitter?
…that is much older than Twitter:
This symbol is an abbreviated version of the earlier /sarcasm, itself a simplification of <\sarcasm>, the form of a humorous XML closing tag marking the end of a “sarcasm” block.
If anything, I would guess probably reddit since it kind of evolved from the early forums where it came from. Twitter has (had?) a much larger userbase though, so that’s not a bad guess either. 🤷♂️