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You can use any unicode characters that have equivalents in the chosen character set.
There is no warning if use a character without an equivalent; it will be omitted at the receiving end.
CGA character set
PET character set (actually C64 but it's pretty close)
To enter characters not on your keyboard, copy and paste from the above charts.
All characters on those charts are supported,
whether or not they render properly in your browser,
including those from this extension.
Your browser may also allow entering unicode directly.
For example in Chrome enter ctrl+shift+U, then hex digits, then 'enter' to input a unicode character by number.