A workaround for box-shadow not passing through to the inside of a border

This is a problem more easily shown than explained. Take the following image:

It’s an indicator — the sort you’d find on any colorpicker — with a shadow. It seems simple enough, but how would you build it in CSS?

My first port of call was a box-shadow on an element with a 2px white border. The problem with that is that box-shadow’s shadow doesn’t take border styles into account, it just creates a shadow behind the element it was applied to resulting in this:

After some tinkering, this is the solution I came up with…

  • Add the white border to the .indicator element as before
  • Create a .indicator::before psuedo-element, and give it a border of the same width, only making this one black
  • Leverage filter to blur the before element in imitation of box-shadow’s blur

…which, once translated into SCSS, does indeed produce a .indicator with a shadow that can be seen both on the outside and the inside of the white border.

// For a `<span class="indicator"></span>` element

$indi-width: 8px;
$indi-height: 8px;
$indi-border-width: 2px;

.indicator {
    border-radius: 50%;
    border: $indi-border-width solid #fff;
    display: block;
    height: $indi-height;
    position: relative;
    width: $indi-width;

    &::before {
        border-radius: 50%;
        border: 2px solid rgba(#000, .5);
        content: "";
        filter: blur(1px);
        height: $indi-height;
        margin-left: -$indi-border-width;
        margin-top: -$indi-border-width;
        position: absolute;
        width: $indi-width;
        z-index: -1;