content -- a DOM CSS2 property/attribute?

Dear Sirs,

Could you help me to put to use "content", a DOM CSS2 property 
(http://www.w3.org/TR/2000/REC-DOM-Level-2-Style-20001113/css.html#CSS-CSS2Properties-content)? 
By this property, I want to substitute an image which is inserted with 
"before" pseudo-element. I use the property in code as follows:

<style type="text/css">
....
[class=MySpan]:before {content: url(image1.png)}
....
</style>

<script type="text/javascript">
....
function Change(A_Span)
  {A_Span.style.content = 'url(image2.png)'
  }
....
</script>

<table>
  <tr><td>Text <span class="MySpan" onclick="Change(this)">Inner text 
1</span>.</td></tr>
  <tr><td>Text <span class="MySpan" onclick="Change(this)">Inner text 
2</span>.</td></tr>
  <tr><td>Text <span class="MySpan" onclick="Change(this)">Inner text 
3</span>.</td></tr>
</table>

The above doesn't change the image in my Netscape 7.2 on Windows98 
platform. Do I everything all right with the "content"?

Or maybe you suggest me something else?

Best regards,
Cyril.