mozilla-release/testing/web-platform/tests/fullscreen/rendering/ua-style-iframe-manual.html

<!DOCTYPE html>
<title>User-agent levels style sheet defaults for iframe</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="../trusted-click.js"></script>
<style>
iframe {
  border: 1px solid blue;
  padding: 1px;
  /* transform is also tested because of https://crbug.com/662393 */
  transform: scale(0.5);
}
</style>
<div id="log"></div>
<div id="ancestor"><iframe></iframe></div>
<script>
function assert_dir_properties(style, propBase, value, state) {
  for (let dir of ["Top", "Right", "Bottom", "Left"]) {
    let prop = propBase.replace('{}', dir);
    assert_equals(style[prop], value, `${state} ${prop} style`);
  }
}

async_test(t => {
  const ancestor = document.getElementById('ancestor');
  const iframe = ancestor.firstChild;

  const initialStyle = getComputedStyle(iframe);
  assert_dir_properties(initialStyle, 'border{}Width', '1px', 'initial');
  assert_dir_properties(initialStyle, 'border{}Style', 'solid', 'initial');
  assert_dir_properties(initialStyle, 'border{}Color', 'rgb(0, 0, 255)', 'initial');
  assert_dir_properties(initialStyle, 'padding{}', '1px', 'initial');
  assert_equals(initialStyle.transform, 'matrix(0.5, 0, 0, 0.5, 0, 0)', 'initial transform style');

  trusted_request(t, iframe);

  document.addEventListener('fullscreenchange', t.step_func_done(() => {
    const fullscreenStyle = getComputedStyle(iframe);
    assert_dir_properties(fullscreenStyle, 'border{}Width', '0px', 'fullscreen');
    assert_dir_properties(fullscreenStyle, 'border{}Style', 'none', 'fullscreen');
    assert_dir_properties(fullscreenStyle, 'border{}Color', 'rgb(0, 0, 0)', 'fullscreen');
    assert_dir_properties(fullscreenStyle, 'padding{}', '0px', 'fullscreen');
    assert_equals(fullscreenStyle.transform, 'none', 'fullscreen transform style');
  }));
});
</script>