There are not many aspects of web design that seem to ignite the same fascination in developers as making elements dynamic by hiding and showing them on user interaction. Collapsible lists, maps with hover elements and multi level drop-down navigations still seem to be hot and need to be part of a web site to make it "cool" and to "increase usability". Much like the magician conjuring the rabbit out of the top hat for the tenth time in a row, this design stunt does gets a bit stale though. Maybe it is time to take a step back and look at what we do.
Dynamic Page Elements-Cloak and Dagger Web Design - Tooltips and Hidden Extra Information (Page 8 of 10 )
One example of hidden and shown elements are "tooltips" that appear when you hover over a navigation or an imagemap. Eric Meyer [note 16] lead the way for this functionality with his "CSS only popups" [note 17] which turned out not to work on some browsers unless you create your CSS the right way [note 18]. They also have some other problems:
1. Content overload
As these tooltips re-use already existing markup, there shouldn't be any content overload. We simply need to turn off CSS or scripting to see if the extra information is really helping or just represents a "nice to have".
For example, a navigation with spans inside each link which get positioned and shown outside the link via CSS can be a nice effect, but the same amount of text in each link might render the navigation hard to follow. HTML already has the title attribute for extra information that could be displayed as a tooltip [note 19].
2. Mouse independence
Per definition, these effects are mouse over effects. However, as they are an enhancement and not a necessity, not much is lost if they don't work with alternative access.
3. Giving false hope
One common mistake is to style the tool tips as if they were clickable and yet hiding and showing them with a hover effect. Users will try to reach the tips with their mouse and get frustrated when they vanish.
Design Usability 
HTML5 Boilerplate Introduction
