Em is generally known as the most effective font unit in CSS content development, because it is relative to the body text size of the page. So, each of your elements is sized relatively according to your current font size. Adding these units to margin and heights of divs will allow a proportional sizing, again according to the text size. In extend, this method will promote accessibility in your page.
Browser testing is a very important part of realizing a successful CSS project. Internet Explorer though is still the most popular browser on the web, splitting between versions 6 and 7. It is also a big pain. Although, version 7 is dominating as the time goes by, there’s still a handful of people using IE6. There are several engines on the web that can provide you screen shots of your design in IE6 for free, but due to traffic it can delay quite a bit.