When I add an h2 tag inside my header div it shifts the whole container div.

Example:

<div id="container">
<div id="header">
<h2>Test</h2>
</div>
<div id="content"></div>
<div id="footer"></div>
</div>

I'm not sure what you mean by "shift", but H2 tags have a margin automatically added (creating surrounding space).

You're also probably triggering margin-collapse (not a bug). Try adding 1px of top padding to #header and see if that solves it.