LayoutTests/fast/custom-elements/defined-rule.html

<!DOCTYPE html>
<html>
<head>
    <title>Custom Elements: The ':defined' pseudo-class applies to elements that are defined.</title>
    <link rel="author" title="Ryosuke Niwa" href="mailto:rniwa@webkit.org"/>
    <link rel="help" href="https://w3c.github.io/webcomponents/spec/custom/#the-defined-element-pseudo-class-defined">
    <link rel="match" href="reference/green-box.html"/>
</head>
<body>
    <style>
        .box {
            display: block;
            width: 100px;
            height: 25px;
        }
        my-defined-element { background: red; color: green; }
        my-defined-element:defined { background: green; }
        my-defined-element:not(:defined) { color: red; }

        my-undefined-element { background: green; color: red; }
        my-undefined-element:defined { background: red; }
        my-undefined-element:not(:defined) { color: green; }

        failed-element { background: green; color: red; }
        failed-element:defined { background: red; }
        failed-element:not(:defined) { color: green; }

        div.box { background: green; color: red; }
        div:defined { color: green; }
        div:not(:defined) { background: red; }
    </style>
    <p>Test passes if you see a single 100px by 100px green box below.</p>
    <my-defined-element class="box">FAIL</my-defined-element>
    <my-undefined-element class="box">FAIL</my-undefined-element>
    <failed-element class="box">FAIL</failed-element>
    <div class="box"></div>
    <script>

        customElements.define('my-defined-element', class extends HTMLElement {});
        customElements.define('failed-element', class extends HTMLElement {
            constructor() { throw 'bad'; }
        });

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