The "if" attribute value associated with the "null" element type must not contain the "<" character

I have a double condition to do a test on my progressmeter. This is the first condition I use:

<zk if="${item.progres <= '33'}">
  <style> span.z-progressmeter-image{ background-image:url('images/vert.GIF'); } </style>
</zk> 

      

        
        
        
      

    

When I run the program, I have this error:

The value of the "if" attribute associated with the element type "null" must not contain the "<" character.