It "just works" if you use JBoss Tools :)
Using eclipse, code complete does not work for xhtml pages. just rename all your xhtml pages to jspx and change the jsf default suffix in your web.xml to reflect that:
<context-param> <param-name>javax.faces.DEFAULT_SUFFIX</param-name> <param-value>.jspx</param-value> <!-- .xhtml --> </context-param>
You may also need to wrap your jspx page content in a <jsp:root> tag, mine looks like this:
<jsp:root xmlns:jsp="http://java.sun.com/JSP/Page" xmlns:ui="http://java.sun.com/jsf/facelets" xmlns:h="http://java.sun.com/jsf/html" xmlns:f="http://java.sun.com/jsf/core" xmlns:a="http://richfaces.org/a4j" xmlns:s="http://jboss.com/products/seam/taglib" version="2.0"> <html> ..... </html> </jsp:root>
And to get code complete to work, just add the s.tld file into the WEB-INF and you'll also need the jsf-ui.tld (in the WEB-INF as well) for code assist with the facelet tag library.
Or, as I said, use JBoss Tools plugins for Eclipse in which code completion for tags (and much more like EL completion into Seam components) just works!