CA doesn't work in XHTML editor if there is <a:loadStyle src="..."/> on the page.

Create Seam EAR Project.
Open home,xhtml
Add <a:loadStyle src="/stylesheet/theme.css"/>
Try Ctrl+Space:

java.lang.NullPointerException
at org.eclipse.wst.sse.core.internal.FileBufferModelManager$CommonURIResolver.getLocationByURI(FileBufferModelManager.java:208)
at org.eclipse.wst.sse.core.internal.FileBufferModelManager$CommonURIResolver.getLocationByURI(FileBufferModelManager.java:188)
at org.eclipse.wst.html.core.internal.htmlcss.URLModelProvider.resolveURI(URLModelProvider.java:401)
at org.eclipse.wst.html.core.internal.htmlcss.URLModelProvider.getCommonModelFor(URLModelProvider.java:86)
at org.eclipse.wst.html.core.internal.htmlcss.URLModelProvider.getModelForRead(URLModelProvider.java:259)
at org.eclipse.wst.html.core.internal.htmlcss.LinkElementAdapter.retrieveModel(LinkElementAdapter.java:239)
at org.eclipse.wst.html.core.internal.htmlcss.LinkElementAdapter.getModel(LinkElementAdapter.java:104)
at org.jboss.tools.jst.web.kb.PageContextFactory$ExtendedLinkElementAdapter.getModel(PageContextFactory.java:933)
at org.eclipse.wst.html.core.internal.htmlcss.AbstractStyleSheetAdapter.getSheet(AbstractStyleSheetAdapter.java:82)
at org.jboss.tools.jst.web.kb.PageContextFactory.getSheetForTagAttribute(PageContextFactory.java:275)
at org.jboss.tools.jst.web.kb.PageContextFactory.createCSSStyleSheetFromAttribute(PageContextFactory.java:238)
at org.jboss.tools.jst.web.kb.PageContextFactory.createIncludedAdditionalInfoForNode(PageContextFactory.java:219)
at org.jboss.tools.jst.web.kb.PageContextFactory.createIncludedAdditionalInfoForNode(PageContextFactory.java:231)
at org.jboss.tools.jst.web.kb.PageContextFactory.createIncludedAdditionalInfoForNode(PageContextFactory.java:231)
at org.jboss.tools.jst.web.kb.PageContextFactory.createIncludedAdditionalInfoForNode(PageContextFactory.java:231)
at org.jboss.tools.jst.web.kb.PageContextFactory.collectIncludedAdditionalInfo(PageContextFactory.java:185)
at org.jboss.tools.jst.web.kb.PageContextFactory.createFaceletPageContext(PageContextFactory.java:154)
at org.jboss.tools.jst.web.kb.PageContextFactory.createPageContext(PageContextFactory.java:98)
at org.jboss.tools.jst.web.kb.PageContextFactory.createIncludedContextFromAttribute(PageContextFactory.java:335)
at org.jboss.tools.jst.web.kb.PageContextFactory.createIncludedAdditionalInfoForNode(PageContextFactory.java:208)
at org.jboss.tools.jst.web.kb.PageContextFactory.createIncludedAdditionalInfoForNode(PageContextFactory.java:231)
at org.jboss.tools.jst.web.kb.PageContextFactory.collectIncludedAdditionalInfo(PageContextFactory.java:185)
at org.jboss.tools.jst.jsp.contentassist.JspContentAssistProcessor.createContext(JspContentAssistProcessor.java:100)
at org.jboss.tools.jst.jsp.contentassist.FaceletPageContectAssistProcessor.createContext(FaceletPageContectAssistProcessor.java:68)
at org.jboss.tools.jst.jsp.contentassist.XmlContentAssistProcessor.createContext(XmlContentAssistProcessor.java:1)
at org.jboss.tools.jst.jsp.contentassist.AbstractXMLContentAssistProcessor.computeCompletionProposals(AbstractXMLContentAssistProcessor.java:85)
at org.jboss.tools.common.text.xml.contentassist.SortingCompoundContentAssistProcessor.computeCompletionProposals(SortingCompoundContentAssistProcessor.java:234)
at org.eclipse.wst.sse.ui.internal.contentassist.CompoundContentAssistProcessor.computeCompletionProposals(CompoundContentAssistProcessor.java:303)
at org.eclipse.jface.text.contentassist.ContentAssistant.computeCompletionProposals(ContentAssistant.java:1832)
at org.eclipse.jface.text.contentassist.CompletionProposalPopup.computeProposals(CompletionProposalPopup.java:556)
at org.eclipse.jface.text.contentassist.CompletionProposalPopup.access$16(CompletionProposalPopup.java:553)
at org.eclipse.jface.text.contentassist.CompletionProposalPopup$2.run(CompletionProposalPopup.java:488)
at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:70)
at org.eclipse.jface.text.contentassist.CompletionProposalPopup.showProposals(CompletionProposalPopup.java:482)
at org.eclipse.jface.text.contentassist.ContentAssistant$2.run(ContentAssistant.java:376)
at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:35)
at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:134)
at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:3855)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3476)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2405)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2369)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2221)
at org.eclipse.ui.internal.Workbench$5.run(Workbench.java:500)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:493)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:113)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:194)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:368)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:559)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:514)
at org.eclipse.equinox.launcher.Main.run(Main.java:1311)