Первая мысль - ха, изи, но:
https://codepen.io/NakaVot/pen/gOpLqOE
В спецификации W3C нашёл:
A box's backgrounds, but not its border-image, are clipped to the appropriate curve (as determined by ‘background-clip’). Other effects that clip to the border or padding edge (such as ‘overflow’ other than ‘visible’) also must clip to the curve. The content of replaced elements is always trimmed to the content edge curve. Also, the area outside the curve of the border edge does not accept mouse events on behalf of the element.
Значит, только свг :)
