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chromium/third_party/blink/web_tests/external/wpt/css/css-values/rem-unit-root-element.html 

<!doctype html>
<title>CSS Values and Units Test: rem units on the root element</title>
<link rel="help" href="https://drafts.csswg.org/css-values/#rem">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  :root {
    font-size: 50px;
    margin-left: 2rem;
    padding-top: 2rem;
    line-height: 2rem;
  }
</style>
<script>
  let rootStyle = getComputedStyle(document.documentElement);
  test(() => assert_equals(rootStyle.marginLeft, "100px"), "rem based margin.");
  test(() => assert_equals(rootStyle.paddingTop, "100px"), "rem based padding.");
  test(() => assert_equals(rootStyle.lineHeight, "100px"), "rem based line-height.");

  test(() => {
    document.documentElement.style.fontSize = "initial";
    let initialFontSize = parseInt(getComputedStyle(document.documentElement).fontSize);
    document.documentElement.style.fontSize = "3rem";
    assert_equals(getComputedStyle(document.documentElement).fontSize, 3*initialFontSize + "px");
  }, "Check that rem font-size is based on the initial font-size.");
</script>