http://zkfiddle.org/sample/2bp62ak/2-sclass-is-not-appling-on-div-if-visible-false
<zk>
<style>
.es-droptargetpreview

{ display:block !important; width:50px; height:50px; border:1px solid red; }

</style>
<div id="test" sclass="es-droptargetpreview" visible="false" />

<button onClick="test.setVisible(true)" label="it work if setVisible(true)" />
</zk>

The expecting behavior is the es-droptargetpreview css class should show up on the div ,
and the css class should work for it in this case.

If works if we have a button to setVisible(true) later.

It looks like kind of ROD mechanism , but I think it's not a good idea,
since we might need to access the sclass from javascript or css in some cases.