Element of
HTML Document Structure
What does <isindex> do?
The <isindex> element was used to create a single line search prompt for querying the contents of the document. Implementation of the element was inconsistent and the functionality duplicated by the <form> and <input> elements. As a result, <isindex> was deprecated in HTML 4.01.
Display
none
Null element
This element must not contain any content, and does not need a closing tag.

More than obsolete

The <isindex> element has been obsolete since at least the previous version of HTML. It was never really implemented consistently across browsers. The idea was that you would place the element into the <head> of a document, and it would let the browser know that you wanted to provide a search function for the document. Or, using the action, a search form for another document. Today, every document is searchable because browsers include a Find feature (CTRL + F or CMD + F in most browsers). And if you want to search other documents, there's a way to do that, too.