Thanks!
One thing is that the chat input box has to be at the bottom, so I believe I need to use position:absolute.
I don’t know how to make the sidebar a preset amount. I can’t change the left content to width:100% - (sidebar width).
I tried settings margins. That worked until I’m resizing the window - the sidebar gets pushed out.
So, I tried javascript…
window.onresize = function(event) {
alert("hmm");
document.getElementById("left").style.width = screen.wdith - 140;
}
I managed to crash firefox by doing that 
Fixed the wdith typo, didn’t work. Hmm.
Then I realized, I need to get the size of the current window, not the screen!
document.getElementById("left").style.width = window.innerWidth - 140;
So, currently: http://glados.cc/chat/layout.html
At first I thought the javascript approach won’t work because the CSS will reset the width. Now I know that the CSS sets the styles when the page is being rendered and doesn’t do anything after, so I can use javascript to change things 
Again, thanks for your help! If I didn’t get a email notifying that there was new replies I probably would have given up on CSS.
By the way: if you see
if(window.innerWidth < 600){
document.getElementById("right").style.display = none;
}
I’d like to hide the sidebar if the width of the window is less than 600 pixels to conserve space. As I haven’t set left to absolute that should give some more space. However the code isn’t working, and I don’t know why?
When I use css to directly set display:none, only the content isn’t displayed. I’m not using position:absolute for left or right, so why does it happen?
From http://www.w3schools.com/cssref/pr_class_display.asp:
none The element will generate no box at all
But when I set css to display:none it still takes up space.
Next next step (the way harder step): actually building the chat and the ajax for receiving new messages. (: