How to hide all elements apart from id=mainContainer - css

Question

I've been trying to hide everything apart from the main content on the following Facebook post
I've been injecting the following css without luck - can someone please help?
html body * {
display:none;
}
#contentArea {
display:block;
}
Below is a screenshot of what I'm after.

Answer 1

With body * you are hiding every child.
With #contentArea you are showing this block, but still - body * persist for child elements AND parent elements.
You have to specify much more rules to hide everything else.

Answer 2

As mentioned before, you cannot display an element which has a parent that was hidden. Anyway, Facebook's layout is simpler than I thought, all you have to do is hide two elements: the header and sidebar. This of course assumes that a user is not logged in.
Inject this CSS
#pagelet_bluebar, #rightCol {
visibility: hidden;
}
Result:
Result (user logged in):
To hide the chat sidebar, you can add #pagelet_sidebar to the CSS.
#pagelet_bluebar, #rightCol, #pagelet_sidebar {
visibility: hidden;
}
To conclude: Hide the main parts instead of everything, or use jQuery to target all except your element as suggested by #MaVRoSCy.

Answer 3

Thanks everyone - the following seems to be the combination of everyone's answers:
#leftCol, #pagelet_bluebar, #rightCol, #pagelet_bluebar {
visibility: hidden !important;
display: none !important;
}
html ._5vb_.hasLeftCol #contentCol {
border-left: initial !important;
margin-left: initial !important;
padding-left: initial !important;
padding-top: initial !important;
}
._5vb_, ._5vb_ #contentCol {
background: none !important;
}