I edited a zhtml file based on a standard HTML/CSS layout. I set the ZK namespaces:
<html xmlns="http://www.zkoss.org/2005/zk/native"
xmlns:h="http://www.w3.org/1999/xhtml"
xmlns:zk="http://www.zkoss.org/2005/zul" xml:lang="en" lang="en">
The layout is only shown correctly layouted in Visual Editor only for some seconds and afterwards the css seems to be disabled.