While giving your site some personal touch, you might wish to have hover-over-popups that describe to the user what he should expect to find behind that link he is interested in.
While there is a standard way to do that (<a title="Hit this!">a link</a>), you might want to
- make the popups come up directly without delaying 2 seconds or so because the user wants to know what is behind that link immediately
- style those popups your own way, not that peculiar yellow the browsers use for that... and you want to do it the CSS way.