I'm not terribly experienced in HTML or CSS but I'm getting there slowly but surely. Recently I've been customising a tumblr blog but I have an issue with IE9. It seems the page is rendered correctly at first but when the CSS changes sweep down the page a lot of the titles etc disappear. I've searched through the code so many times trying to figure it out but no luck. I've been forcing IE8 compatibility mode which seems to fix the issue but a side effect is I lose navigation bar button styling.
Any help would be much appreciated.
The site is as follows: