Not impossible with CSS but better to not do this with CSS. Curves/wave shapes like that are better done with SVG. If you still wish to do with CSS, try searching on css-shapes tag with "wave" or "double curve" as keyword and you'll find a couple of examples.
– HarryMar 18 '16 at 10:08
2
http://stackoverflow.com/questions/29372054/css-3-or-svg-wave-in-a-background, http://stackoverflow.com/questions/27785114/how-can-i-make-a-full-width-wave-in-css, http://stackoverflow.com/questions/17202548/wavy-shape-with-css
– HarryMar 18 '16 at 10:24
@JohannesJander: Dupe closing is to make sure information is not split across multiple questions. The other question says "CSS or SVG" and so anybody wishing to post a CSS answer can still post it there and information will be maintained in one place. Also, there is enough other wave shape questions (you can see a few others in my comment above) which could have been used for closure but I chose the one which is the closest and would be more helpful for OP.
– HarryMar 18 '16 at 10:33
@JohannesJander: I don't have any problems with a disagreement. Its for the community to decide now. If they decide I chose the wrong one and reopen (good) or if they see the point in what I did and don't reopen (still good).
– HarryMar 18 '16 at 10:40