Application compatibility between IE6 to IE8

A few days ago, one of my customers asked me to identify their web application's compatibility issue.

Actually, the problem was that the 3rd party ActiveX control is not shown in IE8, even though it works perfectly in IE6.

Can you guess what prevents it from being shown in IE8?

I turned off every rule related with ActiveX security but it didn't work.

Well, in IE8 there was some script error indicating that some object below the ActiveX control cannot be located.

So, we look into the code and tried to move the elements from below the control to above the control...

Then the script error disappeared...

Hmmm... let me tell you why this happened.

The problem was that some element has not complete form. A <param> element didn't have closing tag or </param>.

Yes, IE6 worked even with incomplete elements without a closing tag, but IE8 doesn't interprete the incomplete element.

After we corrected the html code, it worked perfectly.