I think it's cerated via Photoshop as an image and then simply added to CSS of the web page as background-image of some element (i.e. a
I think the question is more how to build such image in Photoshop?
It might be some new Photoshop effect because I'm starting to see it too often on websites. (It happened the same when PS6+ added the Warp effect, and we started to see on the web many sites with a curl on the up right side of the page.).
Here is a Photoshop tutorial on how to make similar image to the one you asked but with stright angles, I think you need to look for these kind of tutorials.