The basic way this works:
The element that will get a popup help should have an attribute called “help.” The content of that attribute will be the content of the balloon. E.g. <p help=”This is some <em> help</em>.”>
Declare the elements getting a balloon to be position:relative.
Create the help balloons using the CSS styling you’d like, but make their position absolute. Use margin-top and margin-left to position them relative to the element they’re explaining. Don’t forget about negative margins.
Change the mouseenter event for elements with the “help” attribute. Also the mouseleave event.
Remember, I probably did some things very very wrong. Use Qtip or some other library that works and works reliably. But, having written this for myself, I figure I ought to share it. (And, by the way, the Google Books -> Harvard Library code is here. Have pity on a poor amateur.)