Why does IE9 distort my site (developed in IE8)

I have 775 line of handwritten CSS code that essentially draws squares. I don't know which part to post but here is the before and after of the IE9 (IE8 comp view) ef开发者_高级运维fect.

Please let me know and I can post any of the relevant XHTML / CSS code. Previous post here for similar issue and post here trying to understand if Microsoft publishes the parser differences.

Before (working)

After(broken)(IE8 comp view) - Account Menu moved and invisible because it is white. All tab moved. Buttons moved.

They have both pretty different parser so you cant rely on anything where IE8 makes something correct IE9 makes it wrong and the other way around.