Flipping text upside down was surpisingly easy and widely supported accross diffrent programs. You could use some kind of CSS transformation like rotate(180deg) to get a visually perfect upside down text. It would be visually perfect, but not exportable.
Instead I looked through Unicode's less known characters and found upside-down versions of most text characters, which is what I'm using here. That way, if you copy the output text and paste it somewhere else (like a social network, or an email) it will be pasted upside down. No CSS, no JS, no tricks, just unusual characters from the Unicode.
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Well, obviously I coudn't find a suitable character for every letter in the alphabet in both uppercase and lowercase, but I got all the lowercase letters, a few uppercases, and a few numbers. You should be able to write a full sentence without bumping into missing characters.