HTML and CSS Reference
220.127.116.11. The title and longdesc attributes
Like most other standard tags, you can provide a title for a frame with
the title attribute. The value of the attribute is a quote-enclosed string
that describes the contents of the frame. Browsers might display the
title, for instance, when the mouse passes over the frame.
If the title attribute isn't quite enough for you, you can use the longdesc
attribute. Its value is the URL of a document that describes the frame.
Presumably, this long description might be in some alternative media,
suitable for use by a nonvisual browser.