Try to copy text and paste text in textarea.
You have noticed on Twitter that when you go for a Tweet there it show How many characters are remaining to complete 140 characters and it changes on every character input.You have noticed on Twitter that when you go for a Tweet there it show How many characters are remaining to complete 140 characters and it changes on every character input.
Not only on Twitter there are many other websites that use the same feature. For example – In the medium, you could see this in Bio section there it allows 160 character input after that it didn't take any inputs.