Index - Web APIs
usvstring is equivalent to domstring except for not allowing unpaired surrogate codepoints.
... unpaired surrogate codepoints present in usvstring are converted by the browser to unicode 'replacement character' u+fffd, (�).

USVString - Web APIs
usvstring is equivalent to domstring except for not allowing unpaired surrogate codepoints.
... unpaired surrogate codepoints present in usvstring are converted by the browser to unicode 'replacement character' u+fffd, (�).

Mozilla Content Localized in Your Language
add fluency :: spelling, punctuation, capitalization, diacritics, typography, unpaired brackets or quote marks, whitespace.

JS_EncodeCharacters
this happens if the destination is too small for the resulting string or if the source buffer isn't proper utf-16 because it contains unpaired surrogate values.

WebSocket.close() - Web APIs
syntax_err the reason string is too long or contains unpaired surrogates.

WebSocket.send() - Web APIs
syntax_err the data is a string that has unpaired surrogates.