Can't see button text whilst running simple demo

?page id="testZul" title=" New ZUL Title" cacheable="false" 
	language="xul/html" zscriptLanguage="Java" contentType="text/html;charset=UTF-8"?>

<zk xmlns="http://www.zkoss.org/2005/zul"
	xmlns:h="http://www.w3.org/1999/xhtml" 
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
	xsi:schemaLocation="http://www.zkoss.org/2005/zul http://www.zkoss.org/2005/zul/zul.xsd">
	
<window title="Hello" border="normal" width="400px">
Do you like ZK? 
<label id="label"/>    
    <separator/>    
    <button label="Yes" onClick="label.value = self.label"/>    
    <button label="No" onClick="label.value = self.label"/>  
</window>

</zk>