mozilla-release/testing/web-platform/tests/intersection-observer/zero-area-element-visible.html

<!DOCTYPE html>
<meta name="viewport" content="width=device-width,initial-scale=1">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="./resources/intersection-observer-test-utils.js"></script>

<style>
pre, #log {
  position: absolute;
  top: 0;
  left: 200px;
}
#target {
  width: 0px;
  height: 0px;
}
</style>

<div id='target'></div>

<script>
var entries = [];

runTestCycle(function() {
  var target = document.getElementById('target');
  assert_true(!!target, "target exists");
  var observer = new IntersectionObserver(function(changes) {
    entries = entries.concat(changes)
  });
  observer.observe(target);
  entries = entries.concat(observer.takeRecords());
  assert_equals(entries.length, 0, "No initial notifications.");
  runTestCycle(step0, "First rAF should generate a notification.");
}, "Ensure that a zero-area target intersecting root generates a notification with intersectionRatio == 1");

function step0() {
  assert_equals(entries.length, 1, "One notification.");
  assert_equals(entries[0].intersectionRatio, 1, "intersectionRatio == 1");
}
</script>