I gotta say...

Implementing custom tool tips on tags seems like gold-plating work to me. All the modern browsers have built-in, pop-up tool tips for tags if you just add your tool tip texts as title attributes:

Tool Tip Example

Check out the Subscribe and Email buttons at the top of the tutorial and you can see the browser-default tool tips in action -- these buttons are actually tags with background images!

I can see a need for custom tooltips if your client insists, but the CSS in this tutorial just makes them look even WORSE to me.