<zk>
<window id="win" title="Window" border="normal" width="600px">
<toolbar mold="panel" align="center" vflex="1" hflex="1">
<toolbarbutton label="Left"/>
</toolbar>
</window>
</zk>

This simple program bring client error on Firefox and IE, because one of client components is null and the style attribute cannot be resolved.
In zk.wpd

setFlexSizeW_:function(e,a,c,b){if(this._orient=="horizontal"){var d=this.$n("cave");e.style.width=d.style.width=jq.px(....

"d" is null

If i remove hflex="1" for toolbar component, all is OK.