I just noticed that the tutorial wiki site is missing its background in all version of Internet Explorer. This is what it looks like in IE 11 (I scrolled, so the top looks cut off but isn't):
All of the other browsers show a gray gradient in the main content sections.
After doing a little bit of poking, you should be able to fix the problem by changing the base linear-gradient property in main.css, line 38 to:
background: linear-gradient(to bottom, #dddddd 0%,#848484 100%);
There's a similar issue with the left nav block on main.css line 627 where changing "top" to "to bottom" should also fix the problem. It might also be a good idea to add a plain background color rule before the gradient styles just to have a fallback for older browsers. I suspect there was a CSS preprocessor or some old styles that never got updated when the linear gradient spec was finalized.
I guess this is a kind of unusual first post on the FISD, but hope it helps!