What are serifs when rounding an image?

Question

css

Rounding the corners of an image using CSS. I noticed that at the poles it is somehow "dirty".
I removed the background and noticed strange serifs:
https://jsfiddle.net/fe3mxgz7/4/
If you replace img with a div, the serifs disappear.
FF doesn't output anything.
Why is this happening and how to deal with it?

Answer 1

The problem only occurs when there is no img src="...", . As an option, use img src or specify something like

<img src="data:image/gif;base64,R0lGODlhAQABAAD/ACwAAAAAAQABAAACADs="/>