<p>I seem to remember reading that it's possible to declare taglib directives such as:</p>

<pre><code>&lt;%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %&gt;
</code></pre>

<p>in web.xml. This eliminates the need to duplicate this directive in every JSP file where the taglib is used. Could someone tell me <em>how</em> these directives can be added to web.xml?</p>

Sorry, you're slightly mistaken.  If a page uses a taglib, you have to have a taglib directive for it on the page. You could place the common taglib directives in an include file that all of your pages include with an include directive, but at compile time the taglib directive has to be there.
I prefer to NOT have the taglib elements in the web.xml, and instead have the taglib directive specify the URI value that is used in the "uri" element in the TLD that is inside the taglib jar file in your WEB-INF/lib.