Tapesty group pick up my question.

It's happy to know that Tapestry group pick up my question about 508 compliance on ALT tag on FormTable. To work a way around, I just use Javascript to assign it when body onload.

for (i = 11; i< 14; i++) {
      myImage = document.getElementsByTagName('img')[i];
      if (myImage != null && myImage != 'undefined') {
         if (myImage.src.indexOf('arrow-up.gif')>0) {
              myImage.setAttribute('alt','Sort Ascending');
         } else if (myImage.src.indexOf('arrow-down.gif')>0) {
              myImage.setAttribute('alt','Sort Descending');
         }
     }
}