When you rollover an image, Internet Explorer will display the "alt" attribute text, while Firefox will more properly display the text in the "title" attribute of the img tag.
A clarification: The
alt="descriptive text" attribute is supposed to be used
only when graphics are not to be displayed -- screenreaders for the visually impaired, search engine spiders, or just plain graphics turned off for speed. The
title="descriptive text" attribute is supposed to be displayed
only when you hover your mouse pointer over the image (or link or many other items), a.k.a. a "tooltip".
Surprise, surprise, MS IE up through 6 (at least, I don't know about 7) does it wrong. If you have an
alt= tag but no
title= tag, IE will display the alt text for a title when you hover over the image. Therefore, if you have an
alt= attribute, you should always have a
title= attribute too, even if it's empty (
title=""). This will keep IE from displaying the
alt= text as a tooltip. There's no reason you can't have the same text for alt and title, but they will usually be a bit different.