third_party/web_platform_tests/intersection-observer/empty-root-margin.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>

<style>
#target {
  width: 100px;
  height: 100px;
  background-color: green;
}
</style>

<div id="target"></div>

<script>
var target = document.getElementById("target");
async_test((t) => {
    var observer = new IntersectionObserver(t.step_func_done((entries) => {
        var rootBounds = entries[0].rootBounds;
        assert_equals(rootBounds.left, 0);
        assert_equals(rootBounds.right, document.documentElement.clientWidth);
        assert_equals(rootBounds.top, 0);
        assert_equals(rootBounds.bottom, document.documentElement.clientHeight);
        observer.disconnect();
    }), { rootMargin: "" });
    observer.observe(document.getElementById("target"));
}, "An empty rootMargin string is interpreted as a margin of size zero");
</script>