P.S.The upcoming border-corner-shape is exactly what you're looking for. Too bad it might get cut out of the spec, and never make it into any browsers in the wild :(
<div id="grey"></div>
<div class="container">
<div class="diagonal">
<h2>Header title</h2>
<p>Yes a CSS diagonal corner is possible</p>
</div>
<div class="diagonal2">
<h2>Header title</h2>
<p>Yes a CSS diagonal corner with background image is possible</p>
</div>
<div class="diagonal3">
<div class="inside">
<h2>Header title</h2>
<p>Yes a CSS diagonal border is even possible with an extra div</p>
</div>
</div>
</div>
<div class="box">
Some text goes here. Some text goes here. Some text goes here. Some text goes here.<br/>Some text goes here.<br/>Some text goes here.<br/>Some text goes here.<br/>Some text goes here.<br/>Some text goes here.<br/>
</div>
<div class="box">
Some text goes here.
</div>
<div class="box active">
Some text goes here.
<span class="border-bottom"></span>
</div>
<div class="box">
Some text goes here.
</div>