i've got this problem in seam projects, but probably this is some general issue:
sometimes when you start idea and open some seam project (for example, the project created from 'registration' sample) that contain xhtm files, the namespace declarations
xmlns:h="http://java.sun.com/jsf/html"
xmlns:f="http://java.sun.com/jsf/core"
xmlns:s="http://jboss.com/products/seam/taglib"
are error-highlighted as unregistered uris, the elements from these namespaces are not resolved etc. It is impossible to make them resolved (if you don't want to set the external resources for these uris manually). Usually you should close the editors and reopen the project to make the uris resolved, but sometimes this doesn't work...