After successfully deleting a tag synonym on https://ru.stackoverflow.com/tags/synonyms, the popup-message shown consists almost entirely of HTML entities.
Found the line in Traducir: https://ru.traducir.win/strings/12445
Tag synonym deleted successfully.
Синоним метки успешно удалён.
С
, the popups sanitise the characters further to at least&#x421
(not sure if#
is HTML encoded as well, but doesn't matter), at which point, it's rendered as a literal string instead of the Russian character (or other encoded character, including quotes, angle brackets, etc.)