How to make a concave css button?

Question

css

How to make a concave element on one side?
PS Through add. an element with position:absolute will not work, under the button there is an image on the background.

Answer 1

htmlbook.ru/css/border-image
You can draw a picture with a border even in svg, but if png - in this case it would be reasonable to approximately correspond to the average size of the buttons so that at different sizes artifacts are not striking when the image is enlarged