Add backdrop support for checkboxes and radio buttons. Differential Revision: https://phabricator.services.mozilla.com/D282501 bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2015488 gecko-commit: e2a8b6a858841d1fa6a7f3e3d9f60ae06a424924 gecko-reviewers: layout-reviewers, firefox-style-system-reviewers, dshin

diff --git a/html/semantics/popovers/popover-checkbox-backdrop.html b/html/semantics/popovers/popover-checkbox-backdrop.html
new file mode 100644
index 0000000..fd5a095
--- /dev/null
+++ b/html/semantics/popovers/popover-checkbox-backdrop.html

@@ -0,0 +1,18 @@
+<!doctype html>
+<title>::backdrop is supported on checkbox input</title>
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=2015488">
+<link rel="match" href="popover-img-backdrop-ref.html">
+<style>
+input:popover-open {
+  visibility: hidden;
+  &::backdrop {
+    visibility: visible;
+    background-color: green;
+  }
+}
+</style>
+<input type=checkbox popover="" id="a" style="direction: rtl"/>
+<script>
+a.showPopover({ modal: true });
+</script>
+