1 files changed, 39 insertions, 0 deletions
diff --git a/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/backdrop-in-flow.html b/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/backdrop-in-flow.html
new file mode 100644
index 0000000000..06f790979e
--- /dev/null
+++ b/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/backdrop-in-flow.html
@@ -0,0 +1,39 @@
+<!DOCTYPE html>
+<link rel="match" href="backdrop-in-flow-ref.html">
+<link rel="help" href="https://fullscreen.spec.whatwg.org/#new-stacking-layer">
+<style>
+dialog {
+    visibility: hidden;
+}
+
+dialog::backdrop {
+    visibility: visible;
+    height: 100px;
+    width: 50px;
+}
+
+#left::backdrop {
+    position: static;
+    top: 100px;
+    left: 100px;
+    background: green;
+}
+
+#right::backdrop {
+    position: relative;
+    background: green;
+    top: 100px;
+    left: 150px;
+}
+</style>
+<body>
+Test that position 'static' or 'relative' for ::backdrop computes to 'absolute'.
+The test passes if there is a single green box.
+<dialog id="left"></dialog>
+<dialog id="right"></dialog>
+</div>
+<script>
+document.querySelector('#left').showModal();
+document.querySelector('#right').showModal();
+</script>
+</body>