The M-Heart-8 puzzle involves a sequence of symbols. I created a font so you can type all of the symbols.
Spoiler alert! Do not look at the font below until you have solved the puzzle above! It is not difficult.
MHeart8 Font (click to show)
The MHeart8 font contains mirror-symmetrical symbols representing the digits, capital letters, and small letters (in English). It also has a few basic punctuation marks ( . ? ! , ' - ) so you can write simple sentences.
You can use this font to make your own enigmas with different words or numbers. If you are making an enigma that is intended to be hard to solve, it is best to keep it very short, just a few well-chosen digits or letters. You can have fun using this font to type anything you want: your name, short messages, birthday cards, valentines, party invitations, clues for a party game or escape room, signs, comics, or stories. It's not a very secret code because it's so easy to decode, but it's still fun. Use this font when you want to write something that looks like a code or a fantasy language at first glance, but, after the reader discovers the trick, it can be decoded very quickly.
Note: Another mirror-symmetrical typeface was invented by Scott Perky in 1909 (US patent 921156), intended for bi-directional boustrophedon writing style.
Free font download (SIL Open Font License):
Source: I first saw the M-Heart-8 puzzle in print, perhaps in the 1980s or 1990s, but I don't remember where. It was popularized in The Simpsons on TV (1998, episode title "Lisa the Simpson") and in The Oxford Murders book (2003) and movie (2008). If you know a source earlier than 1998, please let me know.