If you are unfortunate enough to have to support Internet Exploder (euphemistically known as Internet Explorer), that piece of garbage that MS inflicted on the world, here are some useful links that will help you.
IE7.js, IE8.js and IE9.js are JavaScript libraries to make Microsoft Internet Explorer5/6 behave like a standards-compliant browser. They fix many HTML and CSS issues and make transparent PNG work correctly under IE5 and IE6.
Visit this site for some interesting reading on the above scripts and the (then) imminent release of Internet Exploder 7