UTF-8, I know, is a character encoding—a system that represents letters, numbers, and symbols by sequences of one or more bytes. There are several such encodings competing with each other, which can lead to computers not always displaying things exactly like they should. (I learned long ago that if a random "J" appears in an email from one of my Outlook-using friends, it's probably supposed to be a smiley face.)
But what, exactly, is ♥ ? Explain this nerdy joke to me, please!
In the (non-unicode) Latin-1 encoding, which always takes one byte per character, these three hex values display as the three symbols in red on the T-shirt. Latin-1 is out-of-date now, but before unicode it was very popular, particularly in html, so you still see it with older websites.
Solution
♥ is a garbled heart symbol (♥). Jason gave a more complete explanation:
The heart-symbol is U+2665, and is encoded in UTF-8 using three bytes, with hex values 0xe2, 0x99, and 0xa5.
Thus, the message of the shirt is "I love UTF-8". Incidentally, because Latin-1 only allows one byte per character, it can only encode 256 characters. That's the main reason it's out of date; there are way more than 256 important characters for modern communication, from 🦄 to 猫 to ☃. Why ™ and ¥ were ever considered more important to have on the lifeboat than ♥ is one of those things you probably have to be a grown-up to understand... 🙃
Congratulations to yesterday's solvers Jacob C., Maddy, Yana, Jason, and Kate. Thanks to everybody who made a guess!
Ten years later, Dr. Shapiro revived Puzzle of the Day at Proof School, writing each day's puzzle on a name tag. After 600 puzzles or so, he was just starting to feel normal about students reading his chest all the time when campus closed and the puzzle, like the rest of our lives, moved online. New puzzles are posted daily on school days.
Want to catch up on old PotDs? There's an archive currently containing puzzles from March to November 2020.