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Test Info:

<!DOCTYPE html>
<head>
<script>
customElements.define("custom-element", class extends HTMLElement {
constructor() {
super();
const template = document.getElementById("template");
const shadowRoot = this.attachShadow({mode: "open"})
.appendChild(template.content.cloneNode(true));
}
});
</script>
</head>
<body>
<template id="template">
<style>
.a + .b { color: green; }
</style>
<span class="a"></span>
<span class="b">This should be green</span>
<slot/>
</template>
<custom-element id="foo"></custom-element>
</body>
</html>