I have asked myself, if it is possible to use facelets xhtml files and taglibs from a jar file instead from the whole web application.

The short anwser: Yes, it is possible

The facelets documentation gives a hint how to use the tag lib from a jar file. But you cannot found in the documentation, if it is possible also to use a xhtml file (referenced from the taglib for example). Of course facelets uses the same approach to load xhtml files from a jar file as for the tag library.

Example:
The follow tag lib definition is placed in my JAR project under /META-INF/myProject.taglib.xml

<?xml version="1.0"?>
<!DOCTYPE facelet-taglib PUBLIC
  "-//Sun Microsystems, Inc.//DTD Facelet Taglib 1.0//EN"
  "http://java.sun.com/dtd/facelet-taglib_1_0.dtd">

<facelet-taglib>

    <namespace>http://thomaswabner.wordpress.com/waffel</namespace>

    <tag>
        <tag-name>select</tag-name>
        <source>components/select.xhtml</source>
    </tag>

</facelet-taglib>

The select.xhtml file is placed in the same project under /META-INF/components/select.xhml and contains some facelets definitions (ui component with a selectbox fo example). The point is, that facelets searches the jar archive under the /META-INF directory for the xhtml file.

Now you can simple use the tag in you web application. You only need to include the jar file with the tag library and xhtml file in you web application classpath.

<?xml version="1.0" encoding="UTF-8" ?>
<jsp:root xmlns="http://www.w3.org/1999/xhtml"
          xmlns:ui="http://java.sun.com/jsf/facelets"
          xmlns:waffel="http://thomaswabner.wordpress.com/waffel"
          version="2.0">
  <ui:component>
     <waffel:select/>
  </ui:component>
</jsp:root>