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.
The beta versions of Internet Explorer 4.0 did not support this element
at all - the final version of IE 4.0, however, supported it again.
Because this element is not widely used, the addition of new elements that
would be supported parents of ISINDEX may break some browsers. It is also
not widely used anywhere but in the HEAD section, which some
parsers may take for granted.