Source/WebCore/css/dialog.css

dialog {
    position: absolute;
    inset-inline-start: 0;
    inset-inline-end: 0;
    width: fit-content;
    height: fit-content;
    margin: auto;
    border: solid;
    padding: 1em;
    background: white;
    color: black;
    display: none;
}

dialog[open] {
    display: block;
}