You need to set this property of the center-div: height:auto (you could also add a minimum height: min-height:400)
About your second question with the footer, this is much more complicated. You must do this:
<div id="content">
<div id="content_left">
</div>
<div id="content_center">
</div>
<div id="content_right">
</div>
<div id="footer">
</div>
</div>
I'll give you now the full CSS (because it's not so easy):
.content {position:relative; overflow:hidden;} //hidden overflow just a hack for common issues...
.content_left {height:auto; float:left} //set height to auto (very important)
.content_center {height:300; float:left} //a fixed height also works!
.content_right {height:auto; float:right}
.content_footer {width:100%; height:auto; float:right} //for tests you can also set a fixed height
This solution is also according to other threads on Stackoverflow: Align DIV's to bottom or baseline, How to align content of a div to the bottom?
But, if you experience problems with that, you may do this (my preferred solution):
<div id="content">
<div id="content_left">
</div>
<div id="content_center">
</div>
<div id="content_right">
</div>
</div>
<div id="footer">
</div>
And its CSS:
.content {position:relative; overflow:hidden;} //hidden overflow is just a hack
.content_left {height:auto; float:left} //set height to auto (very important)
.content_center {height:300; float:left} //a fixed height also works!
.content_right {height:auto; float:right}
.content_footer {width:100%; height:xxx; float:left} //you can use any height...
Note that all above solutions works only if you set all the "contents" to float, it doesn't work with absolute values! I found this here: http://wiki.answers.com/Q/How_can_a_parent_DIV_wrap_around_child_DIVs_which_are_floating_left_or_right
This is due to an issue with divs: It's not possible to "tell" a parent div the size! So childs like "content_center" or "content_right" won't tell the "content" how long they are and how long "content" must be. So it's impossible to tell the footer where to align, if you use absolute values for the childs.
So your second question, although it looks trivial, is a very important question, and not easy to solve.
IMPORTANT UPDATE:
I tried to find a solution with absolute now. The problem is, that absolute and fixed are taken out of the regular (text)flow, so their size can't influence the size/positioning of any other element anymore. But we also have to understand that an absolute element still controls all its childs, so we should rather set the childs as relative than the parent (here: "content")! So I finally found the solution, and it's quite weird, because it's almost the opposite thing I suggested above, but that solution was influenced by the posting of others, while following solution is "my own" one (I added a header for demonstration purpose):
<div id="header">
</div>
<div id="content">
<div id="content_left">
</div>
<div id="content_center">
</div>
<div id="content_right">
</div>
<div id="footer">
</div>
</div>
The CSS (the "header" clearly shows, that "content" inherites all positioning to its childs like "content_left", "content_right", aso.):
.header {position:absolute; left:0; top:0; height:100; width:100%}
.content {position:absolute; left:0; top:100; height:auto; min-width:700} //min-width is only voluntary, but quite useful
.content_left {position:relative; left:0; top:0; width:200; height:auto;} //height:auto is important to adapt the height from containing text!
.content_center {position:relative; left:200; top:0; right:200; width:auto; height:auto;} //in the middle element, also auto-width is important!
.content_right {position:fixed; right:0; top:0; width:200; height:1000;} //we set a fixed position, but that won't influence the footer anymore!
.content_footer {margin:0 0 60 0; position:relative; left:0; bottom:-60; width:100%; height:150;} //a fixed height is also okey...but relative position is needed!
//you still need to add margin:0; border:0; padding:0 or similar values for some elements to get a good layout
The important point here is, that you can decide which child element will be the longest one, and set this element's position:relative, while the other may have absolute or fixed. But if you don't know which element will be the longest, all child's positions need to be set as relative. Anyway, I suggest to set all childs to relative (beside fixed if needed), because their parent "content" will set their absolute height-position already correctly, so there's no need for any absolute at all.
I'm repeating myself: Above I wrote it's not possible to tell a parent div the size...actually it's possible, but not if the values absolute and fixed are used. Only if you use the browser standart value (static) or relative, the parent div will be informed about the size of its childs, an therefore the footer is set correctly at the bottom of the page.
Well, my solution works everywhere...even in IE (tested 6.0 and 8.0!) due to the hack margin:0 0 60 0 where the value 60 should be the positive value of bottom:-60. Now we finally got the non-floating crossbrower-solution. ;)
