This can be achieved using a radial gradient, (Example split onto separate lines to make it easier to read)
background-image: radial-gradient(
/* Position the circle at the center, 40px from the top */
circle at center 40px,
/* The center of the radius should be dark */
rgba(0,0,0,0.4) 0%,
/* This is the inner edge of the circle, we transition from dark-transparent between pixels 30 and 31 */
rgba(0,0,0,0.4) 30px, rgba(0,0,0,0) 31px,
/* This is the outer edge of the circle, we transition back from transprent-dark between pixels 34 and 35*/
rgba(0,0,0,0) 34px, rgba(0,0,0,0.4) 35px,
/* Everything outside of the circle should be dark */
rgba(0,0,0,0.4) 100%
);
Where circle at center 40px
defines the position of the circle relative to the parent element (Horizontally centred, an 40px down from the top) bare in mind this is the position for the centre of the circle so you do need to account for it's radius.
And we use very small steps between the gradient to make it look like a solid line rather than a blurred gradient (I find that a 1px
difference helps prevent aliasing on the line and makes everything look much smoother)
You can adjust the size of the circle or the thickness of the line by changing the 30px
, 31px
, 34px
and 35px
values in the gradient.
Working example:
.item {
position: relative;
width: 200px;
height: 200px;
background: url(https://picsum.photos/seed/picsum/200/200);
}
.item:before {
position: absolute;
content: '';
top: 0;
left: 0;
bottom: 0;
right: 0;
z-index: 1;
/* This is the ring itself, you can adjust it's size using the pixel values, leaving 1px differnce usually leaves the best result for smooth edges */
background-image: radial-gradient(circle at center 40px, rgba(0, 0, 0, 0.4) 0%, rgba(0, 0, 0, 0.4) 30px, rgba(0, 0, 0, 0) 31px, rgba(0, 0, 0, 0) 34px, rgba(0, 0, 0, 0.4) 35px, rgba(0, 0, 0, 0.4) 100%);
}
<div class="item"></div>
(This method is browser compatible with pretty much every browser released since 2010)