alt/title

alt is not intended for tooltip at all. That's a mistake with Internet Explorer.

Alt: An attribute used to display some text in the place of the image if the image was not loaded.
Title: An attribute used to show some text in the tooltip above any element when you pass the mouse over it.

Never use alt with the intention of showing tooltip text.

Here's the correct usage of alt and title:
<img src="house.jpg" alt="[image of a small white house]" title="This is my house!">