DIV with image borders - or alternative

I'm looking to jazz up some dynamic content containers for a site, which currently are only divs with background colour.
Problem is of course, it's a square coloured box, which is dull.

Let's say the page bg is white and the div bg is black.
What I want is to make the black div have shark fins / blade like things cutting into it here and there.

- See attached image as describing this is hard -

I thought of somehow using images for the div borders, but have no idea if that is cross-browser compatible. With image borders I can also make the corners rounded, I know there are methods for this, but if I could make the left/right borders images then I could have the shark fins and also rounded corners with images.

I have tried floating divs for this before and getting it to work in all browsers is a nightmare, mainly the annoyances with IE.

The images I can do, no issue, but what's the best way to apply this to a div. The content is dynamic, so the div needs to be able to grow height wise. Width is fixed px.

Or are DIV's not the best option?