Most web sites' comment sections will allow these. I think even Facebook allows tomfoolery like this. f̷̧̨̡̜͚̬̰͕̩̻̳̜̮̫̰̓̏̑̐̇̍͘͝è̶̙̥͚̰͈͚̹̜̼͙͚͌à̴̛̙̣̖̮̰̥̩͚̣̺͋̂̓̓̍͛͜ͅr̶̛͎̬̭̯͙͉̬̞̤̲̞̼͉̣̃̒͗̄̎̋͆̓̊͐͝ͅ ̷̦̬̳̹̦̳̭͓͖͔̺̮̩̓̆̍̊͆͊͜ẗ̵̩͙̼́̂̿́̂̈́̈́̑ḩ̸̨̝͇̖̤̱̠̼̣͈̩͈̰̃̂͂͋̏̇͗͛̓̈́̍̑͑̕̚͜ë̶̥̜̘͙̦̋ ̸̛̟̀̄̌͗́̄ͅų̷̪͙̰̜̟̰̦͇̞̥̜͓͇͊̆͗̆̀͗̀̈́̌̉͆̏͆͜ṫ̸̨̜͈̲̺͖͓̄̍̄̓̏̓̓̎͜ͅf̴̨̼̜̯͇̜̯̹͚͛8̴̧̤͖̠̳̜̤̻̟̏̀̽̃m̷̯̤̳͔̘̣͙̗̰͙͔̦̰̝͑͐͌̓̎̀̇̓̆͂̎̐̽̚͠ą̶̢̻͂̒͛̑̅̈͛͑̈́̓̚͠͝n̴̦̺͖̆̑͗̾̈̀̉̿̑̐̈́͝ͅͅ
I've seen some sites / services (Discord?) filter these out, at least to the point where they don't escape a message's vertical space. I'm sure they're truncated because those messages are pretty big in terms of amount of bytes.
And while they have valid use cases, I can't see it in e.g. comment sections or chat messages. Happy to have someone link to e.g. a Vietnamese comment section showing practical use though.
