User floe has reported this issue on on #jbosstools channel at irc.freenode.net.
STEPS TO REPRODUCE:
(also See the Environment field)
1. Create a fresh JSF project (JSF 1.2 with facelets, FaceletsKickStartWithoutLibs)
2. Add Maven capabilities
3. Close Eclipse
4. Run Eclipse and open pages/inputname.xhtml
ACTUAL RESULT:
Some tags are rendered as tag-names.
EXPECTED RESULT:
Standard KickStart page is rendered.

The IRC log follows:

floe: The JBoss-Tools HTML-Editor shows a weird preview of my xhtmls with half of the tags show. It states the host www.w3.org is unreachable for validation... is that the reason?
floe: however I can connect to the host via browser.. can I enforce revalidation?
floe: can someone pls tell me why JBoss HTML-Editor suddenly is corrupted and shows tag-names of my xhtmls in preview-mode?
floe: hmm for other projects it seems to work. Can a project be corrupted in some way so that preview is broken? Last thing I remembered was that I added maven capabilities.
yradtsevich: floe: hi, could you please send me an examle project where the xhtml preview is broken? Or you can create a JIRA issue by yourself: https://jira.jboss.org/jira
floe: yradtsevich: I just generated a JSF-Kickstart with facelets
yradtsevich: floe: Hi. We are unable to reproduce your issue with Visual Page Editor and m2e.
yradtsevich: floe:Everything is rendered correctly except values of Messages.properties, but this is an issue of m2e.
floe: yradtsevich: hmmmm
floe: yradtsevich: I used a nightly build of jbosstools and m2eclipse from update-dev site... which only work with java 1.5 btw.
max_at_hibernate: floe: why does m2e affect us ?
max_at_hibernate: floe: I mean, why do you think m2e is causing problems ?
floe: max_at_hibernate: I created a jsf-kickstart project... then I enabled maven dependency management...after I restarted eclipse the preview in jbosstools html editor for my xhtmls was broken.
floe: max_at_hibernate: I did the same at home and at work.. same happened.
yradtsevich: floe: Please, send us an example project
floe: max_at_hibernate: But it's the exact same eclipse/plugins configuration.... I have several other plugins (ajdt-1.6.4, epic, fatjar, groovyeclipse, haskell, implementors, log4e, pydev, shelled, soapui, sqlexplorer, subclipse, textng, tptpt, vep-1.4.0, wireframeskatcher)...
yradtsevich: floe: did you use http://m2eclipse.sonatype.org/update-dev/ as m2e update site?
floe: yradtsevich: yes
floe: yradtsevich: I had trouble using both together for weeks so I tried out different combination of both plugins until I figured out that I have to use java 1.5 to run eclipse... but then the described error occured yesterday.
yradtsevich: floe: You have created a new JSF 1.2, JSFBlankWithoutLibs project, is it right?
floe: yradtsevich: JSF 1.2 with facelets with libs
floe: yradtsevich: hmm, I just created a kickstart and it's fine with maven enabled
floe: yradtsevich: Habe to find out what's different there... only thing that comes to my mind is that we refrenceing a parent pom with java 1.6 ...
floe: I was wrong: I COULD reproduce the "MavenBreaksJBTHTMLEditor"-Bug. I created a fresh JSF-Project using Facelets without libs, Then added Maven capabilities
floe: Restart eclipse and open ..pages/inputname.xhml or greeting.xhtml.
yradtsevich: We tried this, everything worked.
floe: I think I know one thing happens, just don't know exectly when. The Maven "Update Project Configuration" (sometimes) breaks my .settings/org.eclipse.wst.common.component file.