 {
        width: 0px;
        height: 0px;
    }
    #flex > div > div {
        width: 100px;
        height: 100px;
    }
</style>

<p>Test passes if there is a filled green square and <strong>no red</strong>.
</p>

<div id="flex">
    <div><div style="background: green"></div></div>
    <div style="z-index: -10"><div style="background: red;"></div></div>
</div>
