Your intro-body class will need to have a explicit height for the container to align to the middle of it. Or your not providing enough info and/or example to understand what it is your trying to accomplish.
I just want to have a text put exactly in the middle of browser window, regardless of the actual window size in px. It should adjust automatically.
In my project this alignment works OK, when I observe the results in Firefox browser.
In BSS application GUI, item sticks to the top.