I just tried to copy paste only the emojis and my comment didn't go through. This current comment had the emojis but apparently they were filtered out. Interesting
There are some limits on characters which can be posted, notably various emoji and some Unicode charactersets.
Though I did find an example of someone achieving bold text on HN (ordinarily only italic is supported via enclosing content in *asterisks*)[1] by way of some Unicode character points.
That's frowned on but not explicitly coded against, mostly because explicitly coding against stuff takes time.
HN uses a lot of soft social pressure for behavioural control. Largely to good effect IMO.
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Notes:
1. Backslash escapes seem a recent addition to that markup. They're automatically added in cases of unbalanced asterisk usage, which is how I'd noticed them.
I have no idea. Binary never seemed to work all that well on HN and I'd rather people have to use their imaginations now and then. It's not quite 100 Years of Solitude but hopefully it's more like No Exit.