Links and Webs - HTML and XHTML: The Definitive Guide - page 361

Older browsers ignore the prompt attribute, but there is little reason not
to include a better prompt string for your more up-to-date readership.
6.6.1.2. The query URL
Besides the <isindex> tag in the header of a searchable document, the
other important element of this special tag is the query URL. By default,
it is the URL of the source document itselfnot good if your document
can't handle the query. Rather, most authors use the <base> attribute to
point to a different URL for the search. [ <base>, 6.7.1 ]
The browser appends a question mark to the query URL, followed by the
specified search parameters. Nonprintable characters are appropriately
encoded; multiple parameters are separated by plus signs ( + ).
In the previous example, if a user typed "insect control" in the search
field, the browser would retrieve the URL:
cgi-bin/quat-query?insect+control
6.6.1.3. The action attribute
For Internet Explorer only, you can specify the query URL for the index
with the action attribute. The effect is exactly as though you had used
the HRef attribute with the <base> tag: the browser links to the specified
URL with the search parameters appended to the URL.
While the action attribute provides the desirable feature of divorcing
the document's base URL from the search index URL, it will cause your
searches to fail if the user is not using Internet Explorer. For this reas-
on, we do not recommend that you use the action attribute to specify
the query URL for the search.