Hlayout cannot align Buttons well (IE6)

<zk>
  <hlayout>
  <vlayout id="mainvlayout" spacing="10px">This is the first block
  of hlayout 
  <button label="Button 1-0"></button>
  <hlayout>
    <button label="Button 2-0"></button>
    <button label="Button 2-1"></button>
  </hlayout>
  <button label="Button 3-0"></button>
  <button label="Button 4-0"></button>
  <hlayout>
    <button label="Button 5-0"></button>
    <button label="Button 5-1"></button>
    <button label="Button 5-2"></button>
  </hlayout>
  <hlayout id="customize" sclass="z-valign-top">
    <button label="Button 6-0"></button>
    <vlayout>
      <button label="Button 6-1-1"></button>
      <button label="Button 6-1-2"></button>
      <button label="Button 6-1-3"></button>
    </vlayout>
    <button label="Button 6-2"></button>
    <vlayout>
      <button label="Button 6-3-1"></button>
      <button label="Button 6-3-2"></button>
      <button label="Button 6-3-3"></button>
    </vlayout>
  </hlayout></vlayout>This is the second block of hlayout</hlayout>
</zk>