Press the little smily face on the post/comment box and go to the “*” tab.
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For example, lemmy.world has these ones:
How do you do this in Voyager?
I checked there, it doesn’t seem to support them.
This is useful
No smily button on Alexandrite UI :(
They’re essentially just  tags, you can embed any image in your comments
Sure, but custom emojis are like collections of these saved for people to use.
Also: they’re sized better.
