The name `html_safe` is very confusing, and many developers (including
me a few years ago) think what that method does is convert the HTML
contents to safe content. It's actually quite the opposite: it marks the
string as safe, so the HTML inside it isn't stripped out by Rails.
In some cases we were marking strings as safe because we wanted to add
some HTML. However, it meant the whole string was considered safe, and
not just the contents which were under our control.
In particular, some translations added by admins to the database or
through crowding were marked as safe, when it wasn't necessarily the
case.
Although AFAIK crowdin checks for potential cross-site scripting
attacks, it's a good practice to sanitize parts of a string potentially
out of our control before marking the string as HTML safe.
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