I noticed on some websites that when I hovered over a link it showed a pop-up box similar to the "alt=" in image files.

Looking at the source code reveiled a "title=" tag in the links, ie:
<a href="blabla.html" title="This page opens in a new window." target="_blank">
Is this tag valid HTML 4.0 and/or also available in NS(4.7,6.0), or is it special to only IE(5.0)?