Actually, the tooltip doesn't work with all browsers since the web designer used the "alt" attribute. The "alt" attribute is not designed to be used as a tooltip, but IE innapropriately uses it, so many web developers proliferate that usage of it.
According to the W3C, the "alt" attribute to the "img" tag, is designed to provide users with "alternate information", should the user have a non-graphical browser. So, if a user is viewing your website in lynx, links, or elinks (among others), they can still navigate, and derive useful information from icons like that.
However, if the intention is to create tooltips for these icons, it's appropriate to use the "title" attribute to the "img" tag. This has a twofold benefit. First, browsers that follow the rules, will also be able to see the tooltip (IE does obey the "title" attribute if it is available). This includes browsers like Mozilla, Firefox, Opera, etc. Secondly, users who are visually impared, and are using aural browsers (browsers that speak the page outloud) will be able to make sense of the feature.
For best design, both the "alt" and "title" tags would be used for these types of icons. It allows the greatest readability of the site.
Further information is available on the W3C website. Specifically:
W3C ALT Information
W3C TITLE Information
I've actually been meaning to mention this fact in this group...in an effort to get the cache type icon to follow the same attributes.
