chromium/third_party/blink/web_tests/fast/borders/border-radius-circle.html

<style>
    div#t {
        width: 300px;
        height: 300px;
        border: solid purple;
        background-color: purple;
        border-radius: 150px;
        -moz-box-sizing: border-box;
        box-sizing: border-box;
        margin: 20px;
    }
</style>
<div id="t"></div>