Many times I had errors like:
-tag is not closed(xml error)
-xhtml has duplicated id
Id like to write validator to check is those errors exist in files. I bet there is some facelets compiler that compile xhtml into java classes and than validates it.
Eclipse WTP, although not being the best thing out there, has some validation for .xhtml / .jsp files. You can try the corresponding editor in NetBeans or IntelliJ.
If checking the validity by opening the page takes you too much time (due to slow redeployment), consider sending the page to the servlet container on save - either using WTP dynamic project, deployed on a server, or via the FileSync plugin.