selection-universal-shadow-dom.html

<!doctype html>
<meta charset="utf-8">
<title>CSS Pseudo-Elements Test: Universal ::selection in Shadow DOM</title>
<link rel="help" href="https://drafts.csswg.org/css-pseudo/#highlight-selectors">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  ::selection {
    background-color: pink;
    color: red;
  }
</style>
<div id="host"></div>
<script>
  const root = host.attachShadow({mode:"open"});
  root.innerHTML = `
    <style>
      ::selection {
        background-color: green;
        color: lime;
      }
    </style>
    <div id="target"></div>
  `;
  test(() => {
    const style = getComputedStyle(root.querySelector("#target"), "::selection");
    assert_equals(style.backgroundColor, "rgb(0, 128, 0)", "Background color is green.");
    assert_equals(style.color, "rgb(0, 255, 0)", "Color is lime.");
  }, "getComputedStyle() for #target ::selection");
</script>

chromium/third_party/blink/web_tests/external/wpt/css/css-pseudo/selection-universal-shadow-dom.html