Add additional box-shadow to an element where existing shadow is unknown

Question

I\'m assuming the answer to this question is that it\'s impossible, but I\'m asking anyway in the hopes that someone knows of a clever workaround.

Let\'s say I have the

Answer 1

One solution might be to use a pseudo-element:

.left-inset::after {
    content: '';
    width: 100%;
    height: 100%;
    display: block;

    box-shadow: 1px 0 0 rgba(255,255,255,.25);
}

It's a little bit hacky, but as long as the element hasn't defined the ::after pseudo-element already, it should work.