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;
}

dialog:-internal-modal-dialog {
    position: fixed;
    overflow: auto;
    inset-block-start: 0;
    inset-block-end: 0;
    max-width: calc(100% - 6px - 2em);
    max-height: calc(100% - 6px - 2em);
}

dialog::backdrop {
    background: rgba(0, 0, 0, 0.1);
}

::backdrop {
    display: block;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
}