<!DOCTYPE html>
<html>
<head>
<style>
.test {
    width: 100px;
    height: 25px;
    background: red;
    color: red;
}
#host1, #host2  {
    color: green;
}
#host3 div, #host4 div {
    width: 50%;
    height: 100%;
    background: green;
    display: inline-block;
}
</style>
</head>
<body>
<p>Test passes if you see a single 100px by 100px green box below.</p> 
<div id="host1" class="test"></div>
<div id="host2" class="test"></div>
<div id="host3" class="test"><div>text</div></div>
<div id="host4" class="test"><div>text</div></div>
<script>

host1.attachShadow({mode: 'closed'}).innerHTML = `<style>
    :host::before, :host::after {
        background: green;
        width: 50%;
        height: 100%;
        background: green;
        display: inline-block;
        content: "test";
    }
    </style>`;

host2.attachShadow({mode: 'closed'}).innerHTML = `<style>
    :host(.green)::before, :host(.green)::after {
        background: green;
        width: 50%;
        height: 100%;
        background: green;
        display: inline-block;
        content: "test";
    }
    </style>`;

getComputedStyle(host2).backgroundColor;
host2.classList.add('green');

host3.attachShadow({mode: 'closed'}).innerHTML = `<style>
    :host {
        color: green !important;
    }
    :host::before {
        background: green;
        width: 50%;
        height: 100%;
        background: green;
        display: inline-block;
        content: "test";
    }
    </style><slot></slot>`;

host4.attachShadow({mode: 'closed'}).innerHTML = `<style>
    :host(.green) {
        color: green !important;
    }
    :host(.green)::after {
        background: green;
        width: 50%;
        height: 100%;
        background: green;
        display: inline-block;
        content: "test";
    }
    </style><slot></slot>`;

getComputedStyle(host4).backgroundColor;
host4.classList.add('green');

</script>
</body>
</html>