Designing Web-UI Wireframes? Read This
By greimer on Dec 19, 2006
Sometimes, when wireframes specify the placement of an image on the page, they specify the alt text for that image. First of all, if you do this, kudos for thinking to that level of detail. It makes it easier for people like me, who write the actual code, to know exactly what you want. Plus, alt text is important for accessibility, and thinking of that kind of stuff up front saves headache down the road.
It's been my experience, however, that sometimes when the wireframe designer says "alt text," they actually mean title text, or tooltip text, which is different. Alt text is what appears if and when the image is broken or it's not possible to display the image. Title text is an additional bit of info, typically displayed as a tooltip when the mouse hovers over an active element on the page. For best results, continue to specify alt text, but if you also want tooltip text, then specifically call out "title" or "tooltip" text in your wireframe.