modal-dialog-backdrop-opacity-ref.html

<!DOCTYPE html>
<html>
<meta charset="utf-8">
<link rel="stylesheet" href="resources/dialog.css">
<style>
.backdrop {
    display: block;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;

    background-color: green;
    opacity: 0.5;
}
</style>
<body>
<div class="backdrop"></div>
<div class="pseudodialog">Test passes if you see a green backdrop at half opacity.</div>
</body>
</html>

chromium/third_party/blink/web_tests/external/wpt/html/semantics/interactive-elements/the-dialog-element/modal-dialog-backdrop-opacity-ref.html