I game same result as you with Firefox 67 on Linux. Also, if I click somewhere in the empty space above "believe" in "There's a shunicorn there. If you don't believe me," then hold down left mouse button and drag left to highlight the hidden text, as soon as the "n" at the end of "unicorn" is highlighted, the unicorn emoji gets displayed. I.e I can see "🦄 n" and that seems wrong.