css/css-pseudo/highlight-styling-001.html

<!doctype html>
<meta charset="utf-8">
<title>CSS Pseudo-Elements Test: highlight styling: custom properties are applicable properties in highlight pseudos</title>
<link rel="author" title="Delan Azabani" href="mailto:dazabani@igalia.com">
<link rel="help" href="https://drafts.csswg.org/css-pseudo-4/#highlight-styling">
<link rel="match" href="highlight-styling-001-ref.html">
<meta name="assert" value="This test verifies that ::selection styles can set custom properties and they over-ride the originating element.">
<script src="support/selections.js"></script>
<link rel="stylesheet" href="support/highlights.css">
<style>
    main {
        --x: red;
        font-size: 7em;
        margin: 0.5em;
    }
    main::selection {
        --x: green;
        color: white;
        background-color: var(--x, blue);
    }
</style>
<p>Test passes if the text below is white on green.
<main class="highlight_reftest">quick</main>
<script>selectNodeContents(document.querySelector("main"));</script>