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I know that this may be controversial, and there are other possible solutions to the problem, depending on message handling, such as using HTML comments where they do not harm.
I use !!FUZZY!! so as to mark problematic or untranslated words that need further work. So, marked messages show up in the future and I know that they need treatment, which often takes a long time until it is possible to complete.
translatewiki.net usually removes the !!FUZZY!! markers and either deletes them, probably keeping them in a database field, or moves them to the beginnings of messages, depending on circumstances. That makes it harder for us translators to focus the marked word(s) or phrase(s) when quickly skipping through messages.
Only when using the AJAX editor and not reloading anything are !!FUZZY!! markers left in place. That helps, but only during the current session.