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.

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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.