Transparent border radius outside

Question

I need some help, I have a div with border-radius and I need it to be transparent outside the circle div. I tried with :after and outline. With ":after" the border stayed within the div and with outline I couldn't get it rounded.

Does anyone know the answer ?

CSS :

    div.circle {
        background: black;
        width: 5em;
        height: 5em;
        -moz-border-radius: 2.5em;
        -webkit-border-radius: 2.5em;
        border-radius: 2.5em;
    }
   div.circle p {
        padding: 2em 2em 0 2em;
        color: white;
    }
    div.circle:after {
        content:'';
        display: block;
        position: absolute;
        top: 0;
        bottom: 0;
        left: 0;
        right: 0;
        border-radius: 2.5em;
        border: 4px solid rgba(255, 255, 255, 0.51);
    }

CSS with outline property:

   div.circle { 
    outline: 4px solid rgba(255,255,255,0.3); 

    background: black; 
    width: 5em; height: 5em; 
    -moz-border-radius: 2.5em; 
    -webkit-border-radius: 2.5em; 
    border-radius: 2.5em;
}

What I want: http://giovannigras.be/home/img.png

Answer 1

Use box-shadow instead of border:

box-shadow: 0 0 0 4px rgba(255, 255, 255, 0.51); 

Cause a transparent border will transpare the background beneath,
while if you use the spread value in the box-shadow property you're good to go:

Example demo

Also as suggested by @vals you can go with background-clip to retain the background size into the content-box size model cause otherwise goes into the default border-box.

Docs:
https://developer.mozilla.org/en-US/docs/Web/CSS/box-shadow
https://developer.mozilla.org/en-US/docs/Web/CSS/background-clip

Answer 2

If you want your border to be transparent (or semitransparent), and you are setting a black background, you need to set the background limited to the inner part, so that the border can be seen.

The property for this is background-clip: content-box;

CSS

div.circle {
    background: black;
    background-clip: content-box;
    width: 5em;
    height: 5em;
    border-radius: 50%;
    border: solid 5px rgba(0,0,0,0.3);
}

fiddle