SFC2021 accepts a limited HTML formatting either when inputting the text or when a copy and paste is done. In order to count the characters, SFC2021 removes all HTML formatting tags, whitespaces, special characters that are non visible (like carriage return, tabulation/indentation, new line….) and only counts the remaining characters.
The rich text fields have no limitation concerning spaces. What needs to be respected is the number of characters without spaces.
Tip: Tools such as Word can be used to approximate as much as possible the SFC2021 character counting, only if the formatting is removed (I.e. save as plain text, format Unicode).