Note that it does work correctly at runtime.
1) auto complete does not show it

2) it does show up for <h:selectOneMenu /> and <h:inputText />

3) Problems view:
Unknown attribute (requiredMessage) userInformation.xhtml /<appname>/web/secure/administration L/<appname>/web/secure/administration/userInformation.xhtml Facelets Problem (JSF)
4) I do get suggestions, including “required”, but not “requiredMessage” or even “requireMessage”

Our namespace:
<ui:composition xmlns="http://www.w3.org/1999/xhtml" xmlns:c="http://java.sun.com/jstl/core" xmlns:f="http://java.sun.com/jsf/core" xmlns:ice="http://www.icesoft.com/icefaces/component" xmlns:ui="http://java.sun.com/jsf/facelets" xmlns:h="http://java.sun.com/jsf/html" xmlns:ice-cc="http://www.icesoft.com/icefaces-composite-comps">