Default Button for ImageButton in Panel not working in IE-7 & IE-8

I have a search page which has selection criteria & two image buttons Search & Clear. I want to fire Search Click whenever user enters in any of the selection criteria. I put the search criteria & button in Panel & s开发者_Python百科et the default button to Search. It works fine in Firefox & Chrome but doesn't work in IE7 or IE8. My preference is not to attach keydown event to all the search criteria textbox & dropdown. Any idea what's different in IE 7-8 which is stopping this to work.

ASP.NET version is 3.5

Here is my html hierarchy

Use the Button control instead of ImageButton as stated by this article.