For Emacs (and its web browsers, email clients, etc) this comes up in a concept of “confusables” strings that could easily be mistaken for other strings, usually as a result of Unicode tricks (multiple similar code points from different scripts, or composed versus combined characters, or sometimes ugly tricks with LtR/RtL markers. The code added to emacs was. A library that could be used to detect these probably-misleading tricks, but they didn’t implement a policy for them. The uses I saw of the library fell into the sort of “Danger, Will Robinson! This looks like it might be malicious” type warnings that can be found in most browsers these days.