The markup for the above line looks like this:
<p>I &hearts; HTML.</p>
It's plural because they were assuming you'd use it to represent suits of playing cards. Silly them.

If you don't believe me, use your browser's view source function to see it directly. Notice how some of the special characters are represented by gobbldeygook sandwiched between an ampersand and a semicolon? Those are html tags representing character entities, which get translated into the symbol you see on the screen — look, there's one right now!

For more fun entities, Try these fun articles at E2, Wikipedia, or for the true authoritative but hard-to-read Truth, Read the W3 spec. (W3 is the World Wide Web Commision, the folks in charge of setting the rules you play by on the Web.)