HTML Reference Guide
<ISINDEX> ... ISINDEX
The ISINDEX element indicates that the browser must allow the user to search an index by entering keywords. This legacy element from early HTML is much less common now that complex form capability is possible. Although it can be placed in other areas of an HTML document, it is usually found in the HEAD section.
action - specifies the URL of a script to be used to process the ISINDEX data.
prompt - Indicates an alternate string (different from the default) to be used as the query message for the user.